I nearly ended up marrying someone who is basically a JW, even though I've never been one myself. When me and my JW ex girlfriend finally split a couple of months back, she admitted to me that even if we did get married it would never work out between us, and this would have been due to her commitment to the bOrg. However, when she was going for the whole marriage push months before, she used to tell me that there were many in her congregation who's spouses weren't JWs (i.e. trying to make out that marriages between witnesses and non-witnesses work out......). I don't know if it's some kind of strategy that the WT uses to gain more slaves members by saying that JWs and non JWs can be happily married (and therefore having a potential convert ready to join uo), but at least what she said when we split confirmed my suspicions about how things would have gone if we did wed.
I don't know if that's what will happen to your friend, I'm only speaking from my own experience here. But, as others have said here he may find that her loyalty will always be toward the bOrg, and not him and their marriage. I hope he is willing to understand that.