Should I warn this girl or at least tell her what happened?

[Mollyy]

I used to date this guy a year ago. He broke up with me but like two months later we were kind of talking. It's been like that for a year. On and off 'talking'. Well I got a bf this summer and he backed off but the moment we broke up, he came back sending me 'smilie faces' through text. I'm in college and he's a senior at my high school. Every time I came home, he hung out with me. One time around halloween, I hung out with him twice, both times we watched a scary movie and cuddled. During my Thanksgiving break, we were hanging out. He kissed me and told me how much he missed me and that he really liked me. He went on to say how stupid he has been for ever letting me go and he wish he could go back in time and change it. Well, he hasn't talked to me (doesn't respond to my texts or calls) since that night. And the past two weeks he has been tweeting to this girl nonstop. I can tell they are talking now. He subtweets about how she made his night and she will favorite it. And then, out of the blue, he blocks me on twitter. Now I'm really hurt so I send him a message on facebook asking him what happened. He doesn't respond but unfriends me on facebook.

My question is, should I warn this girl (went to the high school I did) or tell her what happened? I don't want anyone to be treated like that and I'm actually starting to think she was kind of talking to him during thanksgiving when me and him kissed. I don't think he deserves her or really any girl until he stops being so immature. What should I do? He's completely out of my life now.