I am going to just take the OP at her word that they can comfortably afford this and accept that the DH isn't worried about affordability, because obviously if something is beyond budget, it is beyond budget.
But assuming that, I have to say I see a lot of this sort of thing on MN - usually house deposit threads, so a lot more money involved - and posters saying they could afford to but why should they. I get that if you can't afford it, you can't afford it. But I'm meaning the threads where they say they can, but it still seems to be a big nail chew. Ditto inheritance.
DH and I tend to view money as something we can do for our family. Ok, we look after ourselves along the way, and for some people I do understand there is nothing more after that. But this attitude that some people have that they wouldn't leave money to their dc etc, in those cases, what do they see the money as being for? Once you have what you need and a decent taste of what you want, surely there isn't any delight in having it line your coffin? I know some people think children aren't "entitled," (very dirty word!) and no, I wouldn't say they are absolutely entitled, but equally, what else do you really want it for? I look at savings primarily as something they will benefit from, something to keep the wolf from their door, or give them enhanced experiences or security. Once you have that yourself, why wouldn't you want that for them?
I mean, in this case, what is the DH hoping to use it for that would bring him more satisfaction than celebrating his child's seemingly happy match?