20% Rental increase - where do we stand?

Posting the situation on behalf of my mother, so I’ll try to include as many details as I know.

Mother moved into this property June 2013.

In June 2023, at the usual point of renewal, the estate agent advised that the landlord wanted to sell soon. At the time, my stepfather was upstairs in a hospital bed receiving palliative care, so my mother convinced them to delay by a year as moving him would be nearly impossible.

June 2024, she was expecting her notice instead of a new tenancy, except nothing came. She chased etc but just no word from anyone. She’s continued to pay rent in full on time, and quietly exist.

Now (October 2024, rent due tomorrow) they have emailed with a new AST to sign but the rental increase is 19.5%. The landlord is under the impression that this will bring it in line with market rates, however the property is neglected to say the least. He has stuck temporary repairs over major issues for 11 years now.

Regardless, what are her options? I’ve told her to pay the original rent without the increase, tomorrow? As without an AST I understand she is on a rolling contract and any increase would be 30 days notice?

What happens if she refuses to pay the increase (which she can’t afford)? She continues to pay the original amount and waits for her S21?