I'm going to tell my friend I think it's wrong they are cheating
Do you think your friend doesn't know it's wrong?
I think they know it but don't care.
I don't think it's a good idea to tell your friend that cheating is wrong because they probably already know that. You shouldn't stick your nose into other people's relationships because doing so rarely ends well. It will probably just cause problems between you and your friend.
It's okay, I want to warn them about it.
It's not your place to dictate what should be going on in anyone's relationships. You really need to get your nose out of their business.