I agree with Kate, I think the decision as to whether you guys pay for his trip should be up to your FI and I think you should completely support whatever decision he makes. Don't get me wrong, I completely get why you don't want to and you guys definitely shouldn't have to. By not speaking up, he's put himself and you guys in a terrible predicament. I think, though, it is a small sacrifice compared to how happy your FI will be to have him there; especially since no one else is coming from his side. And maybe the bestman can work out SOME way to pay you guys back.