I agree with Husband... make it a nice holiday for both of you!!
I apologize for not being up to speed, but does she know that you know OP will probably be there?
If she does, I would just be honest. I'd say "I have no problem with you going to say goodbye to your friends. I am a little apprehensive that OP will be there though."
Leave it at that.. don't imply anything beyond that (assault).
Not sure how she would react to that, you would know what's best there.
