With the teacher's express consent it can sometimes be helpful to speak to the head teacher to try and negotiate a way of keeping the teacher at work or helping him or her to return to work. In the end it may not be possible to resolve the situation and the teacher may decide to resign or the school may decide to terminate the teachers contract of employment. In this case the teacher may decide to apply for medical retirement anyway asking his GP to complete the necessary forms. A letter to the GP explaining the situation and suggesting that a second opinion from a local psychiatrist may be helpful, but must only be done with the teacher's consent. You can see from the above document that the DfEE have not looked upon stress as an illness.
Stress does have a lasting effect and does cause harm.
It can damage your confidence, energy, your sleep pattern, how you function as a teacher and how you see life in general. One good idea would be for someone to write such a document who has suffered from the illness itself. If this was the case you wouldn't be reading it in 3 minutes flat but more likely in days. Now they threaten teachers suffering from stress that if you don't recover soon you will be sacked. This should NOT be the case.