Ugh. ^^^^
Without knowing her, it’s really difficult for any of us to judge her intentions or what she might do in the future. It’s entirely possible she’s a great person who was dating someone she liked, but after meeting you, she discovered you are a better fit. It happens all the time, guys and girls alike. This doesn’t automatically mean she’s the devil. We honestly don’t know.. you would know this better.
I do agree with the advice to not make a move. If you happen to be interested back, I wouldn’t pursue this until way down the line, once your best friend is well over her, should they break up.. If your best friend really likes her and she breaks up with him to start dating you, then your friendship will definitely suffer. Just based on what you’ve said here, I would be loyal to my best friend. I would avoid being alone with this girl, and if it got to the point where I felt comfortable doing so, I would tell my friend.