I don't think there's really clear precedent for these circumstances. There's what would "normally" happen, but we are streets away from "normal". The government deemed her at too high a risk on Friday, but not on Saturday, but the employer is right - from where they are standing nothing has changed. Care homes are also a high risk environment, and no amount of testing will change that. The combination is potential lethal for all concerned. Given that, this may be a case of frustration of contract - the employer can no longer safely employ her, and that is neither her fault nor theirs. And in that situation I can't see any doctor, whether her own or her employers, saying that it is wise for her to be working in that environment. Nobody is going to want to pin their colours to the mast and say that she's safe when she wasn't safe for the last three+ months.
In your situation I think I might start out with a written communication to the employer saying that, as of 1st August, the shielding scheme as ended and the government have told people to return to work, which is what she intends to do, and she requires the employer to tell her when she should report for work. She might also advise them that shielding is not the same as sickness, and so her doctor cannot provide a fit note because she was never sick. Then confirm that she is happy to attend any occupational health appointments the employer may require, but that she is expecting to be returned to payroll, as is her legal right, whether or not the employer expects her to work until they have made the appropriate risk assessment.
That's what I'd do. But equally I think I might be prepared for them to start looking at how they dismiss her, and I am not entirely sure that they couldn't do so legally if they take care and evidence legitimate concerns about the risk. Because, in all honesty, whilst I totally understand your worries about money, this is a travesty, and people who were at risk last week are still at the same amount of risk, and any employer, especially one in a sector noted for the high degree of risk, would be stupid to ignore the risk to their employees and their clients.