Why the fuck isn’t he looking for a new job so you don’t have to be subjected to this kind of crap? Exactly what is he doing to fix the damage he’s caused? Because just promising not to sleep with anyone else isn’t enough.
I think that you should seriously consider telling your husband that you have zero interest in going to an event where all of his colleagues are going to be looking at you and judging. Let him explain why he’s alone.
But if you do go, completely ignoring her is definitely the way to go. Not even looking in her direction. If you say anything, then you’ve confirmed to her and to everyone else that you’re the explosive, difficult harpy that he has probably painted you as. Ignore her and you’ll be the dignified one, while she’ll be on tenterhooks all evening, waiting to see if you do try and flush her head down the loo. She is not going to have a fun evening, watching you with him, while she’s keeping her distance to avoid you. If her name is mentioned, then you bite your tongue, completely blank facial expression and rise above it. You will be so relieved that you stayed dignified. And it might be satisfying to think about kicking her stupid face in and calling her all the names under the sun, but you do that in your head, on the way home. You are a strong woman. She does not have the power to destroy your life or harm you. You are not going to give her the satisfaction of thinking she does.
And when you do eventually realise that you are, in fact, a strong woman and that this pathetic man does not deserve you, you will be so, so glad you kept your dignity.
ETA I took so long to post, I missed your update on the job situation!