Anyone else think this is a weird friendship?

This girl had a part in ruining my relationship by exposing something I kept from my sign. other. She knew the whole time though & she waited months before telling him and told it in front of a group of ppl and even my ex (and only God knows what else she said about me) I think to humiliate me. I get that he is mad at me for not telling him myself or sooner. What I Don't get is why he is suddenly talking and friends with this girl. It feels like the ultimate betrayal to me. It seems since she knew the whole time and the way she told everyone he'd be mad at her too. It's like she set the whole thing up to hurt me. There's nothing romantic between them. Do you think he is 'friends' to hurt me, to prove he's over me, or he trusts Her now more than me, I don't get it, but it hurts and feels like a slap in the face