I agree with Recovering. COULD something like that happen? Sure. People also cheat on loved ones when there is no addiction involved, at all.
If he gets in with the right group of people in AA, trust me, they will counsel him to stay WELL away from the women in the program. Not that he will never have a friendly exchange with them before or after the meeting--but no heart-to-hearts, no meeting each other for "mutual support" stuff. There's a saying I've heard many times in the rooms, "Underneath every skirt there's a slip." (Women have equivalent sayings--my sponsor used to tell me "The men in the program will pat you on the ass--the women in the program will SAVE your ass.")
All things considered, I think infidelity is more likely for a man who is still drinking than one who is in solid recovery mode. Nothing is ever guaranteed, but it simply doesn't strike me as something that should be at the top of anyone's worry list--a worry list that will get shorter, incidentally, to the extent you pursue your own recovery.