But can I still use the hold bag that is in her name for the flight she was originally going to be on, even if she is not travelling?
No, because she will be logged as a 'no-show' as she will have already taken her flight home and will not be checking in for her flight with the rest of you. OK so you may have her boarding pass, but it will need to be scanned at security and presented by hand (hers) at the gate and onboard the aircraft on embarking.