It's not that big a deal, you can tell her without being really dramatic. Just know she has a bf so nothing will happen. I have a friend who told me she'd like to have me as a bf. I never conveyed the same thing back cause her current bf is my friend. But I don't want a gf anyways atm. Nowadays we are just a "pretend couple," like if some guy is creeping on her, I pretend to be her bf so he backs off.