I think it is pretty straightforward, you tell you friend to dump the cheating dipshit.
Most Helpful Girl
I wouldn't tell my friend to "dump the cheating dipshit" as you say.
Though, I've kind of been in that situation.
With a good friend, we were both on this dating website and talked about it. One day at a party, she told me that earlier that day, she saw a friend's boyfriend online with a profile. She asked me for advise on what to do and I advised her to tell our friend. She did. At first, that friend didn't really react but the next day she had break up with him.
They were already having problems. We didn't tell her what to do but we did give her the facts and we were there to listen if she needed to talk.