I told him that he needed to tell her the truth because either way, she's going to get hurt, but I don't know if I did the right thing. I don't want her to get hurt she doesn't deserve this. I don't know what to do.
If they are both Christian, he shouldn't have been out and getting drunk to the point of losing his mind and fidelity. However, if he did, he needs to confront his girlfriend and tell her immediately. It's very important that he does. It shows more "trust" if he tells her rather than if he hides it, and even if you don't tell her, she will someday find out. And she will also find out that you knew, and did not tell her, causing her to probably get mad at you for keeping it from her.
So tell your friend's boyfriend that he needs to tell her, and do so soon. Tell him if he doesn't, then you will have to because it's your duty as her best friend. He must have enough respect for her to let her decide whether she wants to be with him. If he truly "loves" her, then he will do it.
Just because he cheated on her, doesn't mean she will dump him. The decision will be left up to her. However, he may not be a complete jackass because at least he confronted you about his mistake, to see what he should do, to let it off his chest. Obviously, he is having a hard time living with his actions. And he will for a long time until he tells his girlfriend the truth. It's the right thing to do.
"she doesn't deserve this." He doesn't deserve her.
He doesn't deserve her.