I've been that woman. No, he didn't mistreat my cat or refuse to take me out anywhere, but he was clearly not as dialed in as I was and I made excuses for it. It hurt. A lot. I know that I was prolonging my agony, sort of, but I didn't know how to let go of my feelings. It didn't help to be told that I deserved better. It didn't help to be told that he was a jerk. It didn't help to be told that I was deluding myself. Rationally, I knew that he wasn't into me. I think your friend knows that too. Emotionally, I didn't want to accept it. (I suspect this is where your friend is as well.)
What did help was honesty mixed in with a huge dose of compassion. One of my friends said something very wise: "Knowing that he isn't worth your time doesn't make it any easier to stop wishing that he were." Oh so true. It's likely that she knows the guy's a loser. That is a painful fact. She's trying to talk herself out of that knowledge with some very faulty logic. She's going through a loss. It's not just the guy (who is really no prize), but her idealized version of him and the future she hoped they would have together. She's grieving the loss of feeling like she was the one who changed him (or who could have). She is probably seeing this as her failure and not his.
I don't think you should focus on trying to make her see how awful he is, because that isn't going to help. It's still a loss. Instead, I think you should focus on comforting her for her loss. Tell her that you will be there for her, and then be there for her. Take her out so she doesn't sit around stewing. If she starts talking about him, then acknowledge her feelings, express sympathy, and move on. If she seems really depressed, then you can remind her that while yes, this relationship is over and done, there will be others. This guy not working out does not mean that she will be alone forever. It does not mean that she will never have a wedding, or bear children, or any of that. It's just not going to happen with him.
He sounds like a butt.