I feel for your lad,it sounds as if there's very little come and go in his relationship and that's not a good start going forward.She's basically deprived him of a day in his life he's never going to be able to get back or recall,he's never going to be able to say to his kids or grandkids "I was there,what an achievement,what a day" and that's just wrong.
The next thing you know,she'll be demanding a big flash wedding on her terms and 6 months later it'll be all over...
I'm not being misogynistic here as I believe a relationship is a two way thing and both persons involved should understand each other's desires and accommodate them 'where possible'.I have 3 Daughters (and a Son) and if any of them had done what the lass did to your son,they'd have been told in no uncertain terms of my disapproval towards their stubbornness and been told to respect their partner a bit more than has happened in this case.
Maybe I'm just lucky in that I have a wife who I've been with for 48 years who has never stood in my way and she has a husband who takes a similar stance.
I genuinely feel for his situation,she shouldn't have done that to him,it's wrong on so many levels.
Look after him,he's yours and she can never take that away from him