Seekinghelp....I can understand your feelings. I am not a member of AA....but I have spent a lot of time around alcoholics---both recovering ones and non recovering ones. Assuming that your partner and his ex are of different genders----there are AA meetings of same sex only....all men or all women. It sounds like he is currently attending "open" meetings.
I am wondering how early he is in his recovery?
As worrysome as this is for you---I think that the reality of it is that you get to have an opinion (naturally).....but, you don't get an actual vote on how he conducts his recovery. This is based on the principle of each person "staying on their own side of the street" when it comes to recovery.
You do have the right to make your own decision about how you handle your own opinion or feelings. this is true for any of us in any relationship. This is about your own boundaries...which you have the right to establish for your SELF.
Have the two of you discussed your discomfort with this situation? Trust can be a delicate and sensitive subject in any relationship. trust is important as is good communication with each other on the emotional issues.
It is slow, today, on the forum. due to the holiday---holidays and weekends tend to be slower.
There should be some recovering alcoholics, coming along, who can address your specific question much better than I am able to. They will know much more about the workings of AA.