Really Close Friends

Since coming to Uni, i've met this girl with whom i've become really good friends with. We take the same subject and are also involved in many of the same societies. When we were talking the other night we both realised that we both see each other as almost best friends at uni- but that that is pretty much as far as it's gonna go.
The thing is, that back at home all of my really close friends were guys. I was almost always under the impression that to be that close to a girl you have to be pretty much goin out. Of course, at the moment she doesn't have a boyfriend, but she is starting to get pretty close to this other guy. Is it really possible to be her best friend if she starts going out with someone? Wouldn't the other guy become kind of suspicious about my and her relationship?