Well, it depends on other parts of the situation, I guess. First off, what do you want? Do you want to stay with your someone, or do you have feelings for their friend? Second, is the friend being weird about it, and is it causing problems with you and your someone?
Basically, if you don't have feelings for their friend, and if this is causing problems with you and your someone, reassure them that you don't like their friend, and you won't cheat on them. Then, tell the friend that you're sorry, but you don't like them like that. (It might seem easier to simply say "Sorry, I'm in a relationship", but don't do it. It still offers them hope that if you left their friend, you would date them, and if that's not true, you shouldn't say so.)