Hi! As a public services employee working for an employing hospital whose Occupational Health Department up till recently held 'there was no such thing as stress' (and is now delivering seminars to employees on how to handle it!) I resonate with this issue.
The question appears to be one of 'objectivity' versus 'subjectivity' - fact versus feeling. The employer appears not to be hearing this employee's statement that his/her workload has increased appreciably and s(he) can't keep up (and is subsequently experiencing 'stress' i.e. a sense of failure, hopelessness, exhaustion, depression........)
So three things; as previous posters said, the employer cannot demote without the employee's agreement. If the employee were to return to work and immediately begin a 'time-and-motion' exercise on his/her work which could be presented in black-and-white to management in support of the claim that the work has increased to an un-do-able proportion then management must respond by putting help in place! This exercise would also address the issue brought up by a number of posters about the 'subjectivity' of the sense of stress. Monitoring his/her workload in this way gives the employee a factual picture of how the day's work is done, what takes how much time, etc. This can be very helpful and revealing and move towards a solution of the problem if and when management put help in place.
Hope your friend will be OK. Stress is real, it is insidious and it makes life miserable but there are ways to address it.