Sidney Price and the Prom

Ummm, I have two students living in a homestay/foster type situation while attending High School.

Lexi asked Sidney to prom; he said yes. I have the notification still. Lexi has been lightly flirting with Sidney and they have a minor romantic interest.

However Sidney has just burst into their home to ask the socially awkward foster sister Bonnie to prom. :#
Not sure what is going on or what to do. If she says No, Sidney will get upset; but she can't go when he's already taking Lexi...