How do I react?

Long story short - me and my ex boyfriend broke up 3 months ago. We were living together, he lied to me throughout our relationship and left me for a girl at his work. He continued to hang round and lie whilst he was dating her. I ceased contact but he still had stuff at my flat. 2 weeks ago we met so he could get it back (both ended up drinking). Anyway I know it was a stupid mistake so please don’t tell me but we slept together twice (once sober).
fast forward to the weekend just gone - my fiery friend saw him out with the girl (who I knew) he’s with now. She told the girl about him hooking up with me. He denied it then messaged me asking to tell her it’s not true. I owe nothing to either of them - why’s he getting me involved? And what do I do? I don’t want to get back with me but in a horrible way I want to get him back for his badly he treated me (I know, not the right attitude but I’m human)
How do I react?
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