The terms coach and instructor are confused. Coaches deal with contests, preparation for contests, making training programs, helping their students to achieve medals. Ranks are not a coach's business in the most common sense. They are part of the pedagogical progress of judo as a whole. They are the instructor's or sensei's business. If my ranks were my coach's business, I would never have obtained any rank in judo since I never had a coach. I had several instructors, most of whom fortunately also deserved to be considered sensei.
There is also a difference between control and courtesy, and there is a difference between dan-ranks and kyû. In general federations have bothered little about kyû-ranks. They are concerned about dan-ranks. The procedures are normally totally different. The instructor can award kyû-ranks, but not dan-ranks except for in a few countries such as the US.
It has traditionally been a delicate issue for an instructor to award a kyû-rank to a student who is not his own student. In general, instructors have the courtesy not to mess with another instructor's students. Consequently, a student from another club visiting another club and asking the instructor if he could just promote him to a higher rank is denied such a request. However, there are clearly exceptions where the opposite is not unreasonable. An example of this could be a student from France in the framework of the Erasmus program studying abroad in Sweden for 2 years and becomes a member of a Swedish club whilst also maintaining membership in his home club, or a person who takes up a job in another country and no longer is present or can practice in his home club but maintains membership. In such cases, politeness and communication can prevent most problems. If I were the student, I would keep my home instructor informed of what is happening and few instructors in such cases would have a problem with it. That is obviously different from someone going on vacation for a week to another country and coming back with a different rank. Impossible and totally unacceptable, no, one probably could come up with some highly unusual scenario where justification could be found, but in most cases not very courteous. If during such a brief visit, a highly unusual thing happened, such as the students participating and winning a really highly regarded tournament while taking out two Olympian champions and the reigning world champion on his way to the gold, then yes, being awarded a higher kyû-rank probably would not be unreasonable, but even in such a case, there is no compelling reason why the privilege to do so would not be the home club's instructor, and why this couldn't wait until the student returned to his home country.
Since the instructor cannot award dan-ranks, but only a national exam board can, which fortunately only takes place after meeting eligibility requirements and passing a serious exam, the situation is quite different. An instructor does not really "decide" on dan-ranks, but "prepares" his student, and "takes responsibility for the student to have been duly prepared and ready to pass his exam". Really where exactly that happens is much less of an issue. I know that when I tell a student that he is ready for his shodan, I don't care whether he passes that in Japan, France, Germany or whatever country as long as decent and verifiable standards have been met and he was seriously examined by an authorized national or regional board, unless it would be a case of batsugun. If my student goes to the Kôdôkan kôhaku shiai and throws 24 people in the line-up one after another and returns with 2 or 3 dan ranks higher, I cannot imagine having a problem with it even though in this case there was no additional technical exam involved.
The issues really only start when there is a deliberate breach of confidence or loyalty and the student against the wishes of his instructor does things involving ranks. However, in such case while probably in the majority of cases the student who obviously is far less experienced than his coach is likely at faulty, it isn't always like that. There certainly also exists a plethora of examples where instructors have a large ego or aren't exactly handling in the spirit of judo when it comes to their students. While we all like to embrace the idea of the instructor being a universally noble sensei who walks on water, this is a mere fantasy, both in the West and abroad. Instructors who are wife beaters, molesters, practising pedophiles ... also have or had responsibility for awarding kyû ranks or preparing students for dan-ranks, so in how far their wisdom and elevation above any doubt applies is doubtful. Instructors are humans just like their students are an humans are strange creatures with a lot of personal baggage who do strange things in strange circumstances. Add to that the infamous politics of judo that can show its ugly face, particularly higher up the rank ladder, and you have a perfect menu for an 8pm television drama.
When one considers the higher dan-ranks, the situation can get even far more complicated, and so do the politics. I lived and worked myself in about a dozen countries, what do you expect when it comes to progressing through the ranks ? That when I went back to my first club where I started as a kid to ask permission to get my next dan rank ? Obviously that club had long ceased to exist, those instructors had long died or retired or quit, etc ? Moreover, my first instructor was a 1st kyû and never even obtained a dan-rank, so ... You will always have people who see evil in something and even if they can't they will invent it. I believe that there is a even a term that has been created called "federation-hopping" to suggest that one would solely go to a country to get a rank one supposedly is not entitled to. Does it exists ? I am sure it does exist particularly when people decided to leave an IJF federation to join a fairly new or modest federation where they promptly get awarded a much higher rank that is totally incommensurate with their real or previous rank or get acccepted at a much higher rank because they mislead the new federation about their real rank. That being said, there is also a limit at one can be practically done --in good faith and being reasonable--, especially when one really works or lives in different countries or when the ugly face of judo politics becomes so extreme that there is no end to what a previous instructor, federation, or clique might do to interfere. However, to keep on track, in this thread we are discussing a fairly simple case, but a case that in a modern day and time with lots of international movement, will become quite common. When you have many legal regulation in the EC, for example, that enforce the free movement of people, you can't reverse time and try to impose views that still belong in the 1960s when it comes to international movement. This is clearly also in judo. There is a court judgement relying on EC rules that initiated a chance in requirement of citizenship when it came to participating in national championships. When I still participated in national championships, it was a mandatory requirement to hold citizenship of the country. EJU federation had to remove that requirement some years ago. Since you can work and reside with citizenship of any EC country in another EC country, it became very laborsome for people who were no longer living in the country of which they held citizenship, and were no longer members of that federation, having to participate in that country's national championships, even more so if there existed no direct particiatpion ship, and participating was preceded by multiple selection of pre-tournments. Many federations used to have a citizenship requirement. This has long been removed. It is an example of how modern internationalization affects what is common. Just look at a European soccer or basketball team in the 1960s. All members typically were citizenships of that same country, and most certainly not multi-ethnic. Would you want to enforce those things again on present-day leading soccer clubs like Real Madrid, Manchester United, Bayern Munich and all the others ? You can't turn back that clock. In fact the "selling" of players and all similar things are quite common in judo.
The Georgian Jarji Zviadauri better known under his current identity as the Greek Ilias Iliadis is one of the most published cases in judo, but there are many others. Just a couple of months ago a lesbian athlete moved countries to join her foreign girlfriend and is now part of her new country's national team. This is today's reality, but you would still try and enforce old-day rigid procedures when it comes to judo's good ol' preoccupation of the Holy Dan Rank ?