Fortunately, my brother is a dentist. I take our mother to him about every 6 weeks and he cleans her teeth for her since she doesn't really do anything when she puts that brush in her mouth. Pretty sure my brother would do this at a very minimal charge for others because he is aware that this is a real problem for the general health of these people. He has a heart for the elderly and makes routine visits to the local nursing homes. Inquire at your local facilities.

Tell her that if she bites too
hard on the toothbrush, she’s at risk for breaking her beautiful teeth. I’m sure that she doesn’t want to have to start going to the Dentist & spending needless time & money when she could be spending on doing something else that she enjoys.