has happened to me in previous relationship. we were only young, but we were together for ages and were pretty much married. she cheated on me and i cheated on her on separate occasions. basically, we talked about it and got all emotional, we were both very sorry for our mistakes that weren't worth anything, and we got over it. no one can completely get over this sort of thing, ull always remember the pain and shit, but forgiveness is a big part, if u don't have that, or they don't mean much to u in the first place then its better not to drag things on and kid ur self any longer. if they are really close, then they should talk. like u no skid marks said, they were in a rough patch and maybe she was super pissed off and didn't care for the consequences of her fun, but now that things got better again, she could be genuinely sorry for her mistake and want to make it right again. if the guy really loves her he will listen. if she really loves him she will try hard to win him over again. its up to them. it worked for me for a while.
oh and some one said that this is not jerry springer, its a mountain bike forum. tru, but it is in the off topic part. metallica and set top boxes aren't exactly mountain bike related either