My mom was in the same situation and goes to WDW quite often she said (I talked to her after I posted my original suggestion):
If she is staying at a resort with you she should get a MB as well for the hotel stay, then you can buy her 3 day pass and link it to her
MDE profile that will be linked to her MB that is for the resort you are staying at with her. That would be the least headache.
She asked the band teacher for ticket info and they did not have it, plus she figured because it was a large group trip she couldn't link the ticket to MDE anyway. She just bought the extra days beforehand and linked them. Then my brother's window for FP+ opened at the same time as hers and they had no issues... He just had two MB's so he is going to leave his Band Trip one with his teacher so he doesn't mix them up when my mom goes to retrieve him.