Her grandson should go to the police - first, he should speak to a solicitor or the Citizens advice to find out what his grandmother's position would be, I very much doubt they would prosecute an elderly woman who has suffered multiple strokes, an argument could probably be made her judgement has been impaired due to her illness & her dependence on these people who care for her.
If Social Services are the ones providing the carer, are they willing to change the carer?