Friend's exboyfriend dilemma

So there's this guy, who I've been friends with for 3 years through school. We always talked in lessons, joked around and have banter etc and I'd say I've got to know him quite well. However, he has been in an on/off relationship with my friend. Due to school finishing, my friend and I haven't actually spoken and we've got to the point where we've drifted apart and come September, I will be starting Sixth Form and she'll be starting college. So we're currently in the no man land between being friends, and not being friends. Now things get complicated because her and the guy spilt up a week ago after being together (on and off) for 7 months. She treated him awfully-talking/flirting with other boys, grinding up against other boys at parties, taking other boys' jumpers when she was cold. Then she broke up with him over Twitter. But now I'm torn because it's an obvious 'no go zone' to date your friend's ex-boyfriend, but I've recently starting talking to him and although we're only friends at the moment, I've liked him for the past two years and could be the potential of it going further. I don't know what to do- as I said, my friend and I are hardly talking, they both seem to be over their relationship as she met up with another boy a couple of days after her break-up. So do I continue to talk with the guy, or do I stay clear of being involved with him? I generally am torn because I know how touchy this dilemma is. If my friend gave any indication of 'heartbreak' over their breakup, I wouldn't be talking to the guy (even though we have been friends, as I mentioned, for three years) but she doesn't seem to care at all for him. Help, please.

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