You're still very young, and just learned the golden rule of relationships: if they'll cheat with you, they'll cheat on you. Even if her ex was abusive, she was still dating him when she started hitting on you. She sounds like a "relationship jumper" and likely wanted to have a guy waiting in the wings before officially ending things with that guy. That just happened to be you. It's likely that she wanted to end things with you, too, for a while, but didn't want to do so until she had someone else to fall back on. P gave her the perfect excuse for that. They may look happy now, but believe me when I say that she'll do the same thing to him, too. She sounds very emotionally immature and will likely repeat the same pattern over and over again until she goes to therapy to sort through her attachment issues.

When she eventually cheats on P, because she will, he'll probably reach out to you. He'll tell you that you were right about her and cry over how badly she burned him. He'll probably try to "repair" his friendship with you by blaming her for manipulating him and tearing you two apart. Don't fall for it. Any friend willing to betray you the way he did is not a good friend to have in your life. They both took advantage of you here. Grieve the friendship and don't let him back in. Don't let her try to come back in your life either when this relationship inevitably falls apart. Take this as a lesson and grow from it.