Honestly? No, you should give up.

Because you will never have a sure answer to the questions below:

1. Are you sure that you are not the "backup plan" because now her situation is deemed as 'bad' or 'unstable'? What if the girl made it pass the rough period and returns to good terms with the boyfriend?

2. Are you sure that the boyfriend is never going to change? I have seen some situations when the boyfriend suddenly proposes and the girl 180' turn back into lovey dovey with the boyfriend.

MOST IMPORTANTLY

3. Say if she is into you and got closer while she is in a relationship, what makes you think that she won't do the same to you once you are in a relationship with her and gets into the same rough patch?

So yeah, go figure.