Oh dear....I'm so sorry to hear about this hon. I'll bet you feel like you've been blindsided once again....and truly it does seem that way.

At this point, probably the best thing you can do is to sit at the other end of the table from your mom and her bf and ignore them (or at least ignore him). Try to concentrate on your dad and other members of your party during your meal time.

Maybe at some point someone can talk to your mom and/or her bf and let them know that from now on, he is not welcome where you are concerned.

This situation reminds me of an old saying - "Keep your friends close, keep your enemies closer". You always want to know what's going on with him so that you can make good choices for yourself or at the very least talk with someone who can help you talk to your mom about any issues involving him.

I do hope you have a wonderful birthday dinner with your dad and other family members. Remember, you can choose how you will react to things during this time and don't give them the power over you to make you miserable tonight.