Depending on the particular practice performing the medical, she may be asked to provide some detail on her medical history. This may include issues regarding mental health also. It may be within a specific timeframe or it may be a very grey "have you ever had..." My advice is to be honest and state her case.
However, some very important aspects. The doctor will not be disclosing these to the potential employer, they will simply be assessing whether or not she is currently fit to perform their duties. All the potential employer will be told is yes or no, so there is no opportunity for them to discriminate on the basis her illness 4 years ago. Also, her giving the context to the illness will assist the doctor in realising it has no bearing on ability to perform her duties.
Another point is that if somehow they did get that information and that they didn't select her for employment, that it is discrimination and the Equality Acts apply. Hence, the company would rather not know the specifics, just are they fit for work.
Last, it's also better to tell the truth on the forms, lying about it or hiding it wouldn't be a good idea and may come out at some point. All kinds of legislation is there to protect her in terms of confidential information and equality, so she should be upfront in the medical (if asked).