I also would confront the cheater first, depending on the circumstances. This I know by experience. it happened to me.
Quite a few years back, talking like 20 a guy I was seeing was cheating on me.
My best friend at the time, knew but did not say a word to me. Being friends with my bf at the time she confronted him first. She told him if he did not fess up in a week she would tell me. She said it was up to him to make things right.
She did however tell him that hurting someone was wrong and if he didnt want the relationship he was to come clean.
So you know what it worked as he told me everything. Now when my friend and I discussed the circumstances she explained to me that she did not want to make it worse. By that she meant. sometimes when we are young or even as older adults we tend to blame our friend for the mishap. She explained that one reason was for the mere fact that if she would have told me I would of not believed her and been upset with her. I would of found her to be jealous and just causing trouble.
I believe she is right. Being much much older now, I realize when I look at young relationships that jealousy is a big part of them and girls and guys in that certain relationship do not want them talking to other girls/guys it seems to cause a problem in the relationship between the friends. Not that should cause any problems if trust is there, it just does at that age and I guess it will never change.
So it works better if the cheater is confronted,from my point of view anyway.