Well.. she might get over him, once that happens it wouldn't be a problem.. maybe intill then you two should pretend you aren't together? But still see each other at your house.
And if that never happens.. you'll just have to decide who you want more, if he's really special, you could just decide not to be friends with her anymore, or, if she's a really good friend, you'll have to realize, lots of boyfriends don't last forever..you'll have to decide which you want more..
Like, my boyfriend, if any of my friends had a crush on him, or didn't like him for some reason, I'll just tell them that we can't be friends anymore, or I just won't talk to them for a few days. He's a really special guy.
Good luck~