But if the landlord is unsympathetic, this may be considered Elder Abuse. So perhaps you should check with the agency who can help you.
I expect your local laws require that landlords maintain a habitable property. And so, for instance, an interior temperature of 40 degrees in the winter probably wouldn't meet the definition of habitable. But 68 degrees? That's probably OK.
So the answer (the legality) depends on your local laws.
Also, your mother should check her lease to see what it says about provision of heat and airconditioning.
Hope that helps.