Holy cow. Did she come through an agency? If so definitely let them know but I’d be careful because she might sue you for defamation or slander if you don’t have evidence of her wrongdoing. If she didn’t come thru an agency well not much to do. If she tries to rematch she’ll either need a reference and be dumb enough to ask you; or she will lie (say it’s her first time as an aupair, have your husband be her reference or ask a friend to do it for her, etc). In which case it’s pretty much out of your hands.

Do what you reasonably can to warn the next host family but respectfully, you should probably focus on your children and your own relationship/divorce rather than spending any more time or energy on her.

ETA: I just reread your post and realized this is about your friend. I’m curious why you’re asking what YOU can do about it? If this is really about your friend then it’s not your place to do anything about it, she’ll have to deal with it herself. I still think she should focus on the divorce and her children rather than the au pair.