I'm going to cover for my cheating friend
Are you going to cover for him cheating or are you trying to cover for the relationship?
I'm covering his cheating, I don't want my friend to get into trouble.
While you're doing a good thing by wanting to help your friend you shouldn't be supporting bad behavior. Cheating is wrong and to help someone hide cheating isn't going to help anyone in the long run. Have you thought about just being honest?
It will break up their relationship and we don't need another relationship being broken.
Maybe you will be able to save his relationship for now but if you keep covering your friend's bad behaviour, he's is going to do it again. He needs to learn a lesson so you should be honest about his cheating. Don't cover for him.