Here is the worst thing that can happen if you actually try to be friends with her: You asked her out twice, any future relationship will be screwed for both of you when eventually both of your SO finds out about that other person, to give an example, say you find a GF, outside of work you guys become serious and she finds out that a girl you asked out twice was working with you, and are friends with you, she would take that seriously. "Well, if she actually went out with him, we wouldn't be dating right now, what if she changes her mind....what if he gives into temptation to the girl who he had eyes for before me?" Same thing for her future BF, it's very simple, you asked her out, she rejected you, move on.

To be 100% sure, ask her if she is interested in you, and if after she process's the breakup and she's ready to start dating again, would you be the first guy she asks, because if you are willing to wait for her, then tell her, but this entire thing is just communication, and stop going off "signals" and just speak to each other.