Dating my ex's friend?

Alright I am well aware this is risky business but I am curious what you all thing because I am falling for this girl.

About 5 months ago my girlfriend broke up with me. She had been talking to another guy while we were still dating and she later admitted that she felt like she was cheating after we broke up. We ultimately broke up because she was not allowed to date anyone and her parents were beginning to push her to the edge. She is 15 and I am 17. Just a few weeks after the breakup she got flirty with the guy she cheated on me with but they never actually got together (partially because he was abusive). She is now in a relationship with an aquantaince of mine (from what I can tell) and I only talk to her to maintain good terms although I want nothing to do with her.

Here is where things get tricky. During the flirty phase of our relationship some 6 months ago, her best friend was also forming a relationship with a long time crush. He treated her very badly and she would every so often talk to me about her problems. At one point while my then girlfriend was out of town, my Ex's best friend told me that she liked me and that she would have flirted with me had it not been for my then girlfriend and this other guy that she liked. She recently broke up also so we are both single now and she has been talking to me a lot.

I am wondering, is it morally/socially acceptable for me to go for her? I knew her for 2 years before I even met my ex, but since she and my ex are very close I feel conflicted. I should add that my ex girlfriend is moving out of state in December but will be coming back for prom in may because she was asked by her current Boyfriend.

Your thoughts?
Dating my ex's friend?
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