Help help help

Ok so Ik this is a super bad thing of me but I've had this boyfriend on and off for 2 years and we a month ago broke up and a week after he had a girlfriend. He came back while still with her with a bunch of I love you shit and about how I'm the only person he can see his self loving and having kids with and a future and lalala and so we have been sleeping together for like 2 weeks of there 1 month relationship and have proof and am sick of his lies should I tell her or just walk away?