Most common cause is failed thermostat on the immersion heater but there are plenty of other causes. I would suggest that he gets a plumber out to check.
Possible problems are bacterial/viral growth in the cold water cistern(i.e. legionella) and if it's the thermostat that's gone catastrophic failure of the storage cistern. There have been a few deaths in the news where the cistern was incorrectly supported and thermostat failure led to it being full of near boiling water. The cistern failed and dropped it's contents on the people in bed below. The two most recent that I can remember were a baby girl in the West Country and an old couple somewhere.