I'll put my money on hall sensor, fuel pump or fuel filter.
Start with the filter as that's the cheapest, follow the guides on here for seeing if its the hall sensor and that should also tell you if its the pump.
No wassent my bike. having just paid out £89.00 for a set of throttle cables and spending half a lifetime fitting same think gravity carbs would be a great idea!. British bikes 10 mins and a couple of quid and we were away!.
Tell him to listen fopr the fuel pump either not at all or making unusual noise owning to a blocked filter.
Good old Amals!
dave gs. (whose bike has just doubled in value!!)