I'm going to tell my friend he should come clean to his gf about cheating so I could get a chance with her
Why would you do that? Do you think you would be a better match with her?
I definitely would. For one I wouldn't cheat on her.
Wanting your friend to be honest is fine but you're doing it for all the wrong reasons. You're trying to interfere in their relationship because you're selfish rather than just wanting your friend to be a better person. You shouldn't be interfering in the first place so just leave it alone before you end up ruining your friendship.
It's my friend's choice and I'm helping him out.
You aren't helping him out. You want to get with his girlfriend so you are trying to coerce him. Let them work it out themselves.