I would imagine it falls under other duties (most contracts will have this so anyone can be made to do another job in the event of staff shortages etc). She works in a shop, they are asking her to go on the till.
It is sad she suffers from high blood pressure etc and as you say 'she should be retired'. Maybe she needs to find something else or retire. It seems to me that they will be better options than the former in the longterm.
This is another example of what will happen to us all with the ever increasing retirement age. The future vision of us working until we die to save millions may sound good, but having 70,80,90 year olds doing all manner of jobs will not work in practice. None of us can cheat nature.
For the record, when I was a teenager I worked in a shop in the goods inwards area. They decided to spread my job across the whole workforce which meant I had to go on the till. I told them my job must be made redundant then and threatened them with a solicitor. They made my job redundant and paid me off.