I'm unexpectedly falling for my best female friend, & shes given hints that she likes me what should i do? ?

I'm falling for my best female friend, she has a boyfriend that up until lately, and before I found out he's cheated on her numerous times I was actually good friends with. Here lately I've seen the emotional abuse he puts her through firsthand, and seen her stressed, depressed, and cry a lot! This totally crushes me and is compounded by the fact that I'm falling fast for her and she's made it known in subtle ways that she likes me too. And before some idiot starts spouting off about how I should get my own girl, she's taken, blah blah, I let her know and I'm the kind of Guy that I would never disrespect him by making any moves no matter how big of a dousche he's being SOMEONE HELP ME PLEASE INFO ANYTHIBG!