I always have said one of the biggest difficulties in a marriage is if the two people want to live in different places, this includes different states, not just different countries.
Other big things are if one person wants kids and the other doesn’t, and very different spending and saving habits.
I know many many people in reationships where the two people are from different countries, or the couple might be from the same country, but one of them has a job that moves them a lot. Basically if it is because of a job, the spouse has to be on board in general that they are willing to move. If the assignment is a year or less I think the people can easily live in two different places and visit once a month more or less, but more than a year isn’t a good idea in my opinion, unless they have been married a long time and their marriage is for sure very united and forever.
My husband fairly early in our relationship asked if I was willing to move if his job required it. I said yes. He broke up with the girl before me because she was not willing. We did live apart for 9 months when he was expatted to Colombia. Every month either he came home or I went there. We have moved a few times to different states for his career. However, if I am very unhappy living somewhere he effectively gets another job or position so we can make another move. His job as breadwinner has controlled to some extent where we live, but we still are united about our overall happiness about where we live, and the job does not dictate everything.
Assuming the relationship is a good one that is being considered as a very committed maybe forever relationship I think he should move with her if he is ok with the idea. Give it a try. If it sucks he can come back. Although, the one big question I have is will he be able to easily get working papers in the new country?