I can see his point, I can - the flights being that much cheaper now, it does make sense to book them now and then not go if it's not convenient. But I think he has to look at more than just cancelling the flights when the time comes - he has to realise he will be costing the B&G money too if he doesn't go, which might piss them off a bit; plus there's surely a cost to staying there involved somewhere too?
So it's not as simple as "I'll just book them now when it's cheap and then cancel if it's not right when the time comes and it'll only be £50", it won't.
I'd be pretty unhappy with it, I have to say - I think he should talk more to his friend about the situation, and see what their thoughts are on the matter. I also think he should realise that it might be VERY difficult for you to cope without him, even for one night - you might still be in hospital, for e.g., so he'd need to look after your DD, or all sorts of other eventualities.
It's unfortunate for him, but I think he should make his mind up to it NOW that he won't be able to go. That will save a lot of resentment at the time of your DC2's birth, which will spoil an important time for you as a family. Have a chat with him and point out all the things that could go wrong that will stop him from going in the end - see if he sees the force of your argument without having a sulk about it.