How do I move past the betrayal of a friend?

I am having an issue with a current friend of mine- I cant get over something that I witnessed her do in the past. What happened was a few years ago this friend, we'll call her Lauren, decided to set up her friend since 10th grade, Kristy on a blind date. Now the reason for this was that Kristy was VERY shy and sensitive and had never really dated. I was thrilled when Kristy and this guy, Jason, started going out on dates, fooling around, and hanging out, etc. In her innocence Kristy got her hopes up and fell hard for Jason in a matter of weeks. She began privately making plans for their future, such as how they would commute and see each other when she went to college the following year. After about three weeks of dating, Jason became distant from Kristy. Kristy was distraught, but had no idea what was really going on- that Jason secretly had a crush on Lauren. Kristy decided that a vacation was what they needed, so Lauren, Jason, and Kristy went on vacation to the beach together where Jason admitted to Lauren how he felt. One night while they were all pretty drunk and Kristy was asleep, Jason and Lauren messed around. When they got back Lauren confessed to me what she had done. I reminded her of how hurt Kristy would be if she found out what had happened, and how Kristy looked up to her as her best friend. Lauren seemed to know what she did was wrong, but I knew her being a little drunk was only an excuse. She did not seem to feel genuinley bad for what she did or regret it, nor did she have any plans to be upfront with kristy about it. I told her that it was bad what she did but if it never happened again then it would be forgivable because she was drunk. We agreed. Meanwhile, Kristy still did not understand why Jason was being distant and decided to have a party night at jasons. that night we all drank and spent the night at Jasons apartment. somehow, me and Kristy wound up sleeping in the same bed while Jason and Lauren slept together on the futon. I thought nothing of it and the next morning we all went home. I didn't hear from Lauren for a few days. all the while Kristy was posting sad statuses on facebook in obvious distress. When I finally heard from Lauren again she told me that the morning after her and jason slept on the futon they had totally sober- with me and kristy sleeping right there! In the days since then they had like six times a day! I told lauren that that was wrong and that she should have been upfront about her and jason to kristy from the beggining. Lauren became furious and said that that's why she didn't tell me in the first place, because she knew I would act like this. I completley lost my voice, and the remainder of our time together was spent in silence, other that when Lauren tried to break it by telling me how great the was between her and jason. I didn't hear from Lauren again for a few days and I didn't try and contact her. I umed she was with Jason. Meanwhile kristy had given up on jason and was as confused as I was. A couple days later I got on facebook and saw that not only were alyse and james listed as in a relationship, but there was a picture of them embracing in a kiss as her profile picture. You can only imagine how shocked and devestated kristy was when she logged onto facebook. Lauren, who she had been there for through so much, had betrayed her. what followed was a fight in which I took kristy's side. Lauren seemed indifferent towards Kristy's feelings throughout the fight. after they stopped being friends Kristy went into a depression and I didn't speak to lauren for about 7 months. When I finally did we made up and are friends now. The problem is, I still think about what happened. When I bring it up to lauren she becomes furious so I stopped, but I cant move past it. any advice?