Cheating spouse

Does the thought of your spouse cheating again resonate in your mind ? How have you moved on? Do you constantly look over your shoulder or look for a sign of them cheating ? I have a friend in this situation and I know she will never leave him. I just don’t know what else I can do to be there for her. I’ve recommended couples therapy but she doesn’t wanna do it. He’s 19 she’s 21. Yes he’s in the military.

If you’ve been in the same situation or know someone who was, what kind of friend did you want in this situation when you were going through it ?