Can we still be friends?

My best friend turned boyfriend turned ex-boyfriend recently started talking to this girl, who goes to the same college as him, and he has yet to tell me about it. I know he doesn't HAVE to tell me about it, and I'm feeling like it's too soon for either one of us to start bringing up new boyfriends/girlfriends, but I found out about her anyway, so now I'm hurt that he hasn't told me this, considering we NEVER had the break-up talk. A few weeks ago, he would send me texts about how much he missed me and all the things we would do, and then all of a sudden it just kind of stopped. Then I found out through his friend about this other girl, with proof to back it up, and now I'm standing here like "damn..." Part of me wants to wait for him to tell me about her on his own and the other part wants me to say "listen, i know all about her and i think that we shouldn't be friends right now." How can I work this out so that no one (namely me) gets hurt and our relationship doesn't get destroyed? How should I bring this "new girl" situation up?