Is this jealousy or should i be worried?

My ex of a year and I are very close, he is my best friend and I am admittedly over-protective of him. In may he and I were working things out and on our way to reconciliation when all the sudden he wouldnt talk to me, for like 3 weeks not a single word, then when we finally did start talking he told me his distance was because he had fallen for a girl I used to be friends with. At first I was upset and it took me a while but I am ok enough now to put on a smiling supportive face. I am able to give him unbiased opinions on how he is going to pursue this girl into a relationship as they have all the makings but she wont commit to him. The other day we went out for the first time since june together and had a nice time but then we came onto the topic of her and i had been drinking so I let him have my real thoughts on her. She has a history of being promiscuous and sleeping around on her boyfriends, i know this because we were at one time friends before she offered to blow him while we were together, she also is manipulative and greedy, she only wants what she cant have and then moved on to another, I dont want to see him hurt and when I told him this he told men he was worried too, since she was going back to school. She had even said to him that she wanted a chance to be single at college and run around with her sorority . Then the night turned awkward and he told me he still loved me and cared about me, and then things odd and a little physical, now I am even more against this knowing how he feels but i still want to remain objective for his benifit, not to mention i am leaving for basic training soon, i want him to be happy but i dont want to see him hurt by this girl and to be honest i want him back which seems like a possibilty someday, what shoud I do?