How do I help her?

So my friend Katie likes this guy Thomas, and he said he liked her, but then she went out with our friend Matthew. Thomas said he didn't like her anymore but Katie still broke up with Matthew for him. We went on a field trip and I thought it might work out with her because they were being a lovey dovey and cuddling on the bus. Then we get back to school and he says he still doesn't like her. Katie finds out he was just messing with her and she is fine with it and says now she going to mess with his feelings. (She already doesn't believe in amor because of her parent's divorce.) I don't know how I can help her because messing with him isn't going help her feel better. If she messes with him, then he will mess with seguinte girl and the our whole world with relationships will just be a mess.