What can a wife do if another woman tries to steal her husband?

A mutual friend of ours (G) has a new GF who hits on my husband whenever our friends meet (she does this when her BF isn't looking). After 10 minutes of our first meeting, she tried several times to give her cell phone number to my husband, but he ignored her. Since we've never had any problems with infidelity before, I trusted him. She completely ignores me and our three children and acts as if we aren't there.

At the last meeting, the men had a few beers and she started flirting with my husband again. Unfortunately, this time he went along with it and asked for her cell phone number. We had a few difficult months after that, during which my husband texted her for two weeks (he said she was just a friend) and then stopped when he realized how bad it made me feel. He apologized many times (he said he was drunk and her flirting had made him feel "important") but he'd made a huge mistake and it will never happen again. He seems to really regret it and we are trying to rebuild our relationship.

Now a friend from our circle of friends has invited us to her birthday party, and our friend (G) and his new GF will also be there. What should we do? I don't feel comfortable continuing to have contact with this woman, but on the other hand, I don't want to be chased away by her.

I don't want to come across as the jealous or controlling wife, but I also don't think I can trust my husband. So I'd be interested in the male perspective: Can/should I do anything about it, break off contact with her or risk him leaving me for her?

My husband and I have been together for 20 years, we are in our 40s, we have three small children together and always had a loving relationship with plenty of intimacy (no dead bedroom!).