denby's post above gave some sound advice, has your mum thought about contacting ACAS; www.acas.org.uk/advice
We cannot tell you whether or not your mum should give up work, all we can say is that she needs to get full impartial advice before she makes any decision.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems
Is your mum in a union? If so her rep should be able to offer support in dealing with the bosses and also the way forward.
Importantly, does your mum have an occupational pension that she is either paying into - or frozen if i.e an old one with another employer?
If so, she should dig out the details and then contact them asking for their up to date info on medical retirement. Some of this may already be on line I.e Local Government Pension Scheme.