Well, I'm not that kind of girlfriend, lol. We won't argue, but I'll hear his explanation first. I'll go berserk on him if he gives me a reason to, but if he doesn't, then I'll just let it pass. Next time, he should just tells me whenever he will be with his ex. Also, as long as he's not doing anything foul with his ex.
Aw, that sucks. I don't wanna be her. :/ I feel sorry for her. I think that even if some people are 'good friends' with their ex's, they should know what and what not to do still, because they might hurt their girlfriend's feelings.