when she goes to run a bath, the hot tap works fine when the water is cold, but then stops working as the water heats up. All the other taps and the shower head works fine.
Any reason to why the hot water tap stops letting water out as the water heats up?? Pressure?
If your sister has a combi boiler I would assume that it cannot cope with the flow rate of the hot tap. Try turning up the hot water setting on the boiler, hope this helps