@jcboy,
You might get another property manager though.
This is an emergency situation so to speak, that poor woman has no other choice than to move back and take care of her mother. If the property management cannot acknowledge that and help you out here until you get a new tenant, then I don't think they deserve any commission beyond what they're owed to.
If I were you, I would tell the property manager that they have a choice - either you will pay them until the contract has expired and never use them again, or they show some compassion for the woman's situation and lay off the commission until you have got a new tenant.
There are unforeseen circumstances in everyone's life, if a company cannot see beyond that and try to keep the client, then to hell with them.