That's when you go and find some knockout gay guy no one knows and convince him to take her to the prom and act all attentive and shit. Friend of mine's mother did that when her boyfriend dumped her just before prom. It was hilarious. She walked in with this knockout older guy and swept past the ex without even looking. She made it all mysterious and stuff, too.
Of course, it helps if the young lady has the makings of Mata Hari to pull it off.