You see my friend was dating this girl and he broke up with her for some one else... Well right after they broke up we had sex 2 or 3 times one weekend... Now they are back together and she might be pregnant with his kid but says she wants me... What should I do?

Well, I am assuming you don't want her or you would have said that. I would come clean with your friend before she does. You know if you tell her you don't want her there is a possibility that she will come out with the truth, or her version of it.