What can you do? Nothing. Either tell her or hope she doesn't notice. Then maybe next time don't do stupid stuff and damage property.
The only solution not involving money would be to take the panel off, heat it up, and try to push the dent out from the other side. However there is no guarantee it will be flawless and being 13 I highly doubt you have the mechanical knowledge to do that. Better hope she doesn't need a new panel, body shop work is expensive.