@Lee83, you should have made the guest cancel, as they were the ones in the wrong. Should they refuse (they will lose money if they cancel and never want to) you get Airbnb to do it, but NEVER cancel yourself. Fines, blocks, etc. Lowered ranking. DO NOT CANCEL. Unless something has happened from your end which you think justifies the penalties, NEVER EVER cancel. Airbnb hardly ever undoes the issues which follows because you are expected to read about how the site works and the terms before you start hosting.
Edited: I just read you said ''the person she was booking for'' which makes it a third party booking. Call Airbnb, they will fix this for you!
All the best.