The ballot may have been mailed to the address listed on her voter registration. Or if she supplied a different address, USPS may have returned it back to the county.

1st Class Mail is traceable to some extent (intelligent mail barcode) so the county should know whether it was likely delivered or returned. Unfortunately, front-line staffers may not have access to such information.

As many others mention, too late now unless she can travel back to vote in person. Can't fault the process either, since mail delivery isn't guaranteed, and she had ample time to make other arrangements.