I'm about the last person to recommend anything about girls, but one time there was a girl I liked who was dating someone else. She broke up with the boyfriend and being the nice guy I am, I gave her some space to deal with the loss. She was very upset about it. Well, several days later she started going out with someone else. I thought, "Wow. That was fast." I missed out because someone else caught her on the rebound. Our friendship deteriorated some time later, so in the long run it was no loss. I think catching someone "on the rebound" is unethical, but lots of people do it. If she can't be alone for long, you have a good shot at hooking up with her.