<human>: Provide a brief overview of the text.

My best friend, Dan, is coming to visit me in a few days for Spring Break. We used to go to school together until a couple years ago when him and his family moved. His parents had lost their jobs and found work 700 miles away. While he still went to school with me, Dan met a girl named Sarah and they started dating. Sarah was his first girlfriend and the girl he lost his virginity to. They were 15 at the time and now Dan is 18 and Sarah is 17. 

When he moved, I know he was sad because he was leaving his friends and also the first girl he ever really loved. For the two years he has been gone, they have always kept in touch. He has visited a couple times each year and Dan and Sarah have worked out a long-distance relationship. He still likes her a lot and is looking to move back here for college so he can be with her. 

The problem is, Sarah slept with our other best friend Matt. Matt told me about their relationship and I was honestly very shocked. Matt also made me promise not to say anything. I'm torn apart because I want Dan to know what happened between them but I'm too afraid of the consequences. I don't know if Sarah would do it again and I don't want Dan to move back for a girl who won't be faithful or honest with him. I think if he knew, his decision would be influenced dramatically. I want to be a good friend to Dan but I really don't want Matt or Sarah to know I was the one who told him. I'm not sure who else knows about what happened. 

What should I do? How should I go about telling him or should I not tell him at all? Should I talk to Matt or Sarah first? I'd really like any help or advice you guys can give me. Thanks.
<bot>: Friend coming back to visit after being away 2 years, long-distance girlfriend cheated with best friend, friend might move back here to be with her, not sure if I should tell him or how, please help.