Stay in touch. Stay close. If all else fails and you get friendzoned, remember that girls want a nice reliable guy to settle down with- that's if you're nice and reliable enough to wait those years.
And for the sake of everyone concerned, yes, even her boyfriend, please don't hope for them to break up, not even deep in your heart. If it happens it happens, if it never does then oh well, but don't ever wish for two others to be hurt so badly for your sake.