No, I'm not Catholic.
Plus, we don't know that he cheated. yeah, he probably is, but we don't know that. Why is this important? I say in the last 2 paragraphs of this small novel.
Both people in a relationship are expected to be faithful, loving, and supportive. IT IS NOT HER FAULT that he isn't upholding his end. I don't think it is at all. If someone is getting abused, cheated on, lied to, etc, etc, then the fault for the damage it causes is ABSOLUTELY on the person who does it. No question. All I have been trying to say, is that before/while heaping on that fault, she should examine their relationship for anything other than signs he exhibits that may indicate why he could be cheating. If she can't find a reason or problem in the relationship other than his outward actions, then either a: He's cheating for the sole reason that he's a scumbag, b: She has no ability for introspection and can't see the other obvious signs, or c: He isn't cheating and she's misinterpreting signals.
If someone is cheating on their spouse, there have probably been problems before that. Does anyone disagree?. If you can't see the problems before (when using hindsight, at least), then you're a fucking idiot. Have you been constantly fighting? Has one been saying they feel unloved, unrespected, belittled, etc? Do you show that you value their opinion? Do you notice or acknowledge when they do little things for you? Does he touch you, show you physical affection in a loving way without it being for sex? Does he tell you he loves you? Do you do the same? Has he hit you? Have you hit him? If I was in a relationship where I was constantly nagged, treated disrespectfully, etc, etc, I'd probably start justifying in my mind going elsewhere for love and affection, especially if I feel I'm not getting it at home. It's not right, but that's just how it is.
I talk about "saving the relationship" because there is obviously a reason they got married in the first place. I know I didn't just get married because I was bored and wanted better financial aid for school. And I feel that, if my wife hurt me or betrayed my trust, I'd rather take the hard task of trying to repair things than just walk away from everything. Why? I took an oath. Does that mean it will work? No. Does that mean I should just trust her implicitly after this? No. But IF I still wanted her around because I love her, then it would be very hard work for both of us to move forward and make changes. It would hurt, but so does any marriage, or anything else in life. Everyone expecting a happy marriage without hard work, sacrifice, and some amount of hurt is full of shit. It's not just some Christian mumbo-jumbo, this is very basic psychology. Most people think and act through reaction, as detailed by behaviorist theories. WHY would this be any different in a marriage, or why would the rules that apply everywhere apply differently here? All actions cause a reaction, and if you want to stop the reaction, the action must be stopped. Even if two people are perfectly compatible when they meet, Erickson's theories state that we are in a constant state of personality development. Who says that I'm going to exhibit all of the same personality traits 5-20 years from now that my wife fell in love with? Neither of us are going to be the same people we married, but that's the point of marriage. To grow together, to change together. If we work at it, then the results will usually be better than if we don't.
Or, as you say, she can get out of the relationship. Not saying she shouldn't. I don't hold other people to my standards if they don't share my beliefs. If she thinks he's cheating and leaves the relationship, then fine. Empower yourself and make a clean break. But, if he wasn't cheating and she tries using her unconfirmed suspicions of adultery as the basis of her divorce, then things are going to get very ugly in court, especially if he willingly turns over his phone records and the exonerate him.
Now, and we haven't even talked about this. What if she gets his phone records, etc, etc and he isn't cheating on her (not likely, but still)? You think she's going to think "Oh, so he must be faithful"? Hell no. I've suspected a girlfriend of cheating, and no amount of evidence to the contrary ever dispelled my suspicions. For me, the suspicion grew and crept into the relationship, and she broke up with me because I turned into a douchebag. Granted, I was 15-17 at the time, but still. How is this going to be any different?