Of course we hve reactions to things based on experience and depending on who we are. I'm not saying that she had NO reason to question but that she was pretty abrupt in choosing to go off with another person, and even if it wasn't cheating since they were not a couple, it says a lot about how impulsive she is. Does he want to get into a relationship with someone that is so impulsive? Also she sounds like she used sex with this "friend" to sooth her pain, that's also not a good sign. What if their relationship is rocky for a bit, will she also use sex as a pain killer?