Council say elderly mother with dementia cannot be added to tenancy?

I have a lady tenant on LHA in a two bed house for the last two years. She has decided to move her mother in with her 90 years of age suffering from Dementia.

She had her own council bungalow and all her rent and council tax was paid as she is on pension credit and on higher rate attendance allowance.

I gave my tenant a new tenancy agreement with a rent increase.

The housing benefit office says her name can not be on the tenancy agreement as she is suffering from Dementia and she is not responsible for anything.

My point is that if my tenant keeps her mother without showing her on the tenancy, she can be penalised for keeping a non dependent adult in the house?

I would be grateful for any advice on what is the best way my tenant can keep her mother and at the same time get the full housing benefit?

Many thanks

Ravi