Friend & Prom issue. Help!?

Well I have this friend, who is supposedly my best friend but she treats me like a damn babysitter and she really is the older one but also the most immature person ever. Well I have this guy friend who is shy and very nice lets call him "Mufasa". But Mufasa, this nerdy nice guy, that I met frist because we get along, asked my so-called friend, "Rasputin" to prom. Congrats right? But Mufasa not only asked but PAID $130 for his and her ticket. So Rasputin then tells me and my other friend, "Churchill" that she might not even go. WHAT THE HELL? Mufasa is going to be crushed because she sort of did lead him on. Rasputin is my "friend" but so is Mufasa who did not do anything to deserve that. So what should I do if Rasputin doesnt go? Should I side with Girl over Guy, even if the circumstance are like this?