It cannot be done.
You're just going to have to go and make the best of it.
Whatever excuse you make, they're going to know that you don't want to go. They won't know why you don't want to go and will make assumptions as to your reasons....don't like him, want to split them up , etc etc. Nothing good will come of it.
It's your choice. Go , eat food you are not happy with and keep good relationships with your daughter and her boyfriend, or don't go, save your stomach and offend them
Sorry.
Cannot see anyway round this.