What would you do in this situation?

I would tell the the person cheating that I saw them and tell them to straighten the situation or that I would.

I think I would take a picture of them , then confront the cheater with the pic and tell him/her right in front of the other person (cause a lot of times, they don't know) that you are showing it to your friend.

If I was the one being cheated on I would want to know. I would hope if a friend seen it they would tell me.

I would most definitely tell them, I know it's not my place but I think it would be better for them to know then to not.

i would bring them both together and say what i have to say because at the end of the day that's my best friend

There's more to consider than the criteria mentioned. Usually, I'd keep things to myself. There may be factors at play I'm not familiar with. Maybe they have an arrangement of some sort, etc. The first thing that comes to my mind, as to why I might inform my friend, would be if they were going to be married within a few days, or if I thought the cheater were trying to swindle my friend in some way. As a bartender many years ago, I learned that if you try to help many females, with a bad lover, the woman will turn on the man who tried to help. And tolerate the cheater. Often trying to be a good friend when dealing with a situation in somebody elses relationship, may cause your own friendship to end.

If we are friends I would tell them because they have a right to know. They may never find out if nobody tells th,e because some people are good keeping those from there partner from finding out.

I think I need to tell my friend about it. It is not good to be in silence.

I think you should find a good time to talk to him/her alone and tell it right away...if he/she is your friend he/she will understand and your concern for her... in the moment he/she might not believe you but at least you've warned him/her about it...and if she/he will find out soon, you'll never know how he/she will thank you more.....

@sianadri I would not let my friend to be cheated and so tell them everything.

Let your friend discover it. It is their matter

I would tell him or her. Cheating is a bad thing. Perhaps if you were the gf of that cheating bf, you would like to know the truth as well.

I think is not fair for any one to stay in such wrong relationship therefore I will definitely make my friend aware of that so he can save himself before getting more hurt.

I would tell my friend so they will know and later might be hurt if they found out i knew all.along and did not say anything .. Better to always be upfront and honest ..

I would tell my friend. I know my friends and I know they wouldn't want to be with someone who cheated on them. I see it as just being a good friend. Now I wouldn't just go off and tell them with out proof because they might not believe you if you don't. I just recently had to help a friend out of an abusive relationship. She was in it for 4 years and I told her regularly she needed to leave and it would get no better. The truth is that they won't leave or do better for themselves until they are ready for it. I would still tell them thought because that is just what being a good friend is and just a decent person.

Karma is a b*tch. So, I (probably) wouldn't say anything.