So, my best friend had a thing with this chick, a while ago. And long story short she expressed very boldly to him that she was disinterested.

However, he (despite everyone's input) still thinks that there's a chance and stuff. Now he is stuck on that one chick and refuses to let her go even though she's not there.

Lately, we have been slightly flirting and she has been sending some STRONG signals towards me, however I have been trying to keep it "platonic" for now because I don't want it to affect my friendship with my friend.

Should I keep back and let him chase her? Or should I escalate with her?