Some mistakes done cannot be undone. I might, depending on how deep my love for her was at that point in time. But, of course, I would become more wary. Do unto others what you want others to do unto you. We have to be realistic.
Love is blind.
One may fall blindly in love and sometimes, there is no turning back. However, I must stress on the point that after knowing that she is attached, I will check if their relationship is stable. Most importantly, whether the boyfriend has been treating her well. If their relationship is stable as a bull, I will back out immediately. I will keep a distance from him, but still maintain as friends.