The key thing here to remember here is that she is his x girlfriend so all is fair in love and war! Why not mention to her that you like him and just see what she says, if she is a real friend i wouldn't think she would mind at all, you sound very nice to consider her feelings but you shouldn't let her stop you. Why not ask just this boy if he'd like to go to the movies with you? I should do this when there is just the two of you around if as you say, he is likely to get embarrased. Or, you could say you were thinking of going for a walk later and ask if he would like to join you.