AITA for warning a girl about a guy she’s giving a second chance to?

Am I wrong for warning a girl about a guy she’s giving a second chance to? In February of this year, I started messing around (texting and going out) with a guy, everything was fine, he behaved better than anyone I had met before. We saw each other, fell in love, everything was perfect. However, when we went out at the beginning of May, he was obsessed with me, saying how much he loved me and all that, so that the next day I would see him with someone else and, unfortunately for me, end everything with him. 2 weeks barely passed, I couldn’t believe my eyes what I saw, I started to fall into depression, only to have him call 20 days later and say let’s go out and talk. Since I still loved him, I agreed. Everything was nice and even my friends thought he had changed. But every story comes to an end. So he wrote last night that his parents found out about us and that they are preventing him from seeing each other. I was broken, but he promised that we would write. When I asked him if it was the other girl (in fear of a repeat of what happened in May, even though he promised a million times that he wouldn’t do it), he denied it, that last night around 9 pm he wrote that he loves another and that the end of us. When I read it, I couldn’t believe my eyes, I burst into tears and wrote with my friends about it, in order to find comfort. Not long after that, the girl contacted me with a message that she was the one he was messing with and that she didn’t know we were still writing, to which I replied that she should beware of guys who cheat on the same girl twice. She was shocked and we started writing about him and we both found out a lot that we didn’t even know was possible. She wrote to him that it was over and that he should forget about her, and that he should be ashamed of his behavior. I realize that I warned the girl and I had no intention of ruining their relationship, but he blames me for it, so am I an asshole? Would it be bad if i gave him 3rd chance?