Couldn't imagine falling out with a friend about this. How does that happen?
Could you not? If one player thinks he is being cut short, then it's surely a potential problem
Player A should insist that PLayer B gets 10% of Copenhagen.
Player B should insist that Player A gets 10% of travel/hotel covered.
That's how friends do it.
Player B assumes after event that he has 10% of seat anyway and look foward to the rail.
Player A and B discuss the matter in which A says to B that he indeed does not have 10% as it was not discussed.
Player B asks for opinion of a reputable name in poker who suggests the above and Player B offers 10% of costs.
Player A still says that he isn't giving away 10%.