Who would be more prone to cheat? Someone who is close friends with an ex or with a former crush?

I have a friend who is constantly in arguments with her current boyfriend, because she is close friends with a guy she had a crush on a few years ago. The feelings for him are totally platonic(she realizes he wouldn't have been a good match). Now her boyfriend lives right around the corner from his ex and watches her chlild when she can't find a babysitter on short notice(not his child by the way). He has the nerve to constantly give her grief. I feel as though she is dealing with a lot more, and has to put quite a bit of trust in him. He constantly chastises her about the frienship with her former crush. Whenever he brings it up my friend reminds him that his situation is a lot harder to deal with than hers. He believes that she would be more prone to cheat than he ever would, because he no longer has feelings for the ex. Which situation would be harder to deal with?