The truth is, when it comes to dating and relating, there really are very few black and white answers.
The conversation at dinner only serves to confirm your nagging suspicion.
Listen to the way she justifies his behavior and decide whether she seems open to what you’re saying. See if she’s willing to talk about her concerns, or if she’s just pretending that everything is perfect when it clearly isn’t. She’s not going to find out you don’t like him and immediately call him up to dump him… You have to let what you’ve said sink in, so give her a chance to come to a conclusion on her own.
You can’t control her ultimate decision; all you can do is plant the seed that maybe this guy isn’t quite the Prince Charming she thought he was. It’s reasonable to expect your friend to be angry at you for bringing up negative things she probably already noticed about her boyfriend. Don’t ever give her an ultimatum or say she’s going to lose you as a friend if she stays with her guy. Ultimately, she has to make this decision on her own.
If you’re close with your friends, you have probably experienced numerous “meet my new boyfriend” dinners/casual drinks/coffee breaks.
Your opinion on her new guy is almost as important as her own mother’s, and it goes both ways. He was probably nervous to meet you (if he has any sense), so don’t judge him just because he was a little quiet. That doesn’t automatically mean he did something unspeakable.