Help?

I'm in love with a guy (I'm bi he's straight) and I think my other friend is falling for me. She's hinted at a relationship a few times and were constantly flirting. If she asks me out what do I tell her? Do I say yes to try and move on over him and make her happy or is that unfair on her. Do I say yes but tell her about it so she can walk away if she wants to?