Is it ok for me to date this girl?

Let me start off by explaining things. I had a best friend named Brian, and we had been friends since childhood. He started dating a girl named Alex and I was really happy for him. I had become friends with Alex. Then I found out from Alex that Brian had been cheating on her for the past year with another girl and Brian never told me anything about it. I was upset but I tried to stay out of it but Alex kept telling me more and more things. Like Alex and Brian went out and ran into Miranda (the other girl) and she confessed to Alex that they (her and Brian) had been having . He denied it and Alex tried to believe him but something told her not to. Then later Brian called Alex and said that everything was true and that he was leaving her. But he called back later that day and said that he was just saying that because Miranda was on the phone and he was trying to get back at Miranda. Well anyways after a year of suspicion Alex confirmed the truth and they tried to work things out. They broke up for awhile and got back together one last time. Brian told me that while they were broken up he messed around with Miranda but he didn't think he did anything wrong because they were broken up but he told me that Alex wasn't aware of it. He also did not know that I knew about the cheating. I had known for over two years and he still hadn't told me. Well they broke up for good and then Alex was just talking to me and telling that Brian had confessed everything to her about Miranda and so I umed that he told her that he had messed around with her during their last break period and so I asked her about that he she got furious and said she didn't know. I was thinking oh crap. Well I told her not to tell him because I didn't want to get involved but she told me she had to confront him. She told him that she never wanted to hear from him again. Then he proceeded to blame me for everything and basically said that I betrayed him and that we weren't on speaking terms anymore. He said that he didn't consider me his best friend right now and he wasn't sure if he even considered me a friend. Then he started accusing me of trying to steal his girlfriend. This happened March of this year and I did not hear from him until May. Alex confronted me during this period and all of a sudden Brian came back into the picture after Alex told him once again that she wanted nothing to do with him. He claimed that me and him are best friends again and he wanted me to stop talking to Alex (as a friend he had no knowledge of anything else). I told him that I would not do that because I felt it would be wrong to turn my back on her when he had turned his back on me. Well by telling him that he basically stopped ociated with me. It is now December and me and him have not been on good terms since March and he only speak with each other if we run into each other in public. Alex wants to date me but I was wondering if I should. I like her a lot and I had no feelings for her prior to finding out how Brian treated her so its not like I wanted to steal her from him. Because she expressed interest at the end of 2007 but I didn't do it because I did not want to betray a friend. Me and Alex got really close during all of this time that I have not been ociating with Brian. Part of me is worried that it could cause some major problems if I were to date her. I do like her a lot and she has feelings for me but I am wondering if I should or would it be a betrayal to Brian. On one hand Brian was my friend from childhood but we haven't been friends since March. I am 24 and I had been friends with Brian since 5, but part of me thinks that he proved himself not to be a good friend with the way he acted. One thing I was worried about is that this could cause problems between me and Brian's mutual friends. But a couple of my friends have said that I shouldn't worry about it. So what do you guys think.