OP, if you have a man who is worthwhile , hardworking, productive, kind and successful, he is the type of man to whom many women would likely be attracted. Are you going to be angry and potentially confrontational with all of them? There may be women in his workplace with whom he spends many hours a day who may be attracted to him and who may make him aware of their attraction. They are not likely to be stopped by your engagement or plans to be married. Would you feel as though these women are disrespecting you? They see someone that are attracted to and they are willing to act on the attraction. That happens on a daily basis in work places, bars, hotels, clubs ,etc.
These women owe you nothing, not even respect. They are willing to put their self interest above even their own self respect. If you cannot be certain that your partner loves and is decidedly faithful to you, then you have no relationship worth saving. If your relationship is solid, it does not matter if every woman he meets makes a play for him.
It is likely that his ex may have been inebriated or was realizing that in ending their relationship many things were left unsaid. Realizing the good things that she lost has absolutely nothing to do necessarily with wanting him back.
There are only 2_people who have an obligation to respect your relationship , and that would be you and your partner. The commitment exists only between the 2 of you.
You did not lose anything by the ex's midnight confession, and nothing will be gained by confronting her. If your partner strays it will be because of a decision that he made and not because some women tempted or lured him away.