A quick google search:
http://en.wikipedia.org/wiki/Water_heating
go to the bottom of the page titled, "water heater safety"
...... basically the risk of Legionanaires' disease is associated with water heaters where the temperature is not set high enough (minimum 50 deg C), increasing the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them .
Your mum is using a condensor boiler - completely different set up, so I reckon someone's got their wires crossed.