ADVICE PLZ! Crush likes best friend

Okay so I like this guy a lot, and today I found out that he is going to ask my best friend to prom tomorrow, and I'm happy for her, but I also guiltily want her to say no cause I don't want to be ditched at the prom and we already had plans going together, but everyone else, like EVERYBODY else, is going to be there and they all want to persuade her to say yes. 
And I don't want my friendship with this girl to melt away. I also like this guy and I'm somewhat disappointed that he doesn't even like me.
What should I do? I haven't told my friend this is happening, and when it does, no matter the answer, do I tell her my feelings for this guy? And then I don't want to go to prom if they go together cause she's my only friend I hang out with, so idk... please help! All advice appreciated!!!