Does the engine temperature gauge indicate it is at normal running temperature?
If so feel the two small hoses that go into the bulkhead behind the inlet manifold when the engine is hot. Heater control positions have no bearing on this test.
Are they both hot? Yes, then we can assume a reasonable flow of water through the heater rad and thermostat working ok.
Are they both hot? No, we can assume air in the system or heater rad/pipes blocked. A leaking head gasket can put air into the system and it gets trapped in the heater first, your friend may have bled the air out and all has been ok for awhile, but no cold air would be the fan and its wiring or fuse ect.
If you drive at speed do you feel any warm air coming through even with the fan off? If yes then it is the fan blower that needs looking into.
If the pipes were hot and with the engine running switch on the heater fan, can you hear the fan running? If yes and there is no hot/cold air then heater direction/flow controls could be faulty or in need of adjustment, or.......the fan blade itself has fallen of the motor (rare).
If you cannot hear the fan then yes it could be a fuse or............have you recently heard any sqeaking from the heater when on slow speed?
The bearings are a weak point and can dry out slowing the fan motor and blowing the fuse (DO NOT PUT IN A HIGHER RATED FUSE), in extreme cases the bearing can fail to the point where the fan (looks like a hamsters spinning wheel) tilts and hits the heater casing and stopping completely.
Replacement of the fan motor entails complete removal of the dashboard and heater box assembly, looks like a nightmare but really is quite straightforward, fan motor was about £50.00 last year?
Hope that helps, others will be along with other ideas,
regards
Dave
