Gas tank will not fill

My neighbor has a Toyota truck. The gas gauge says full which may be a sending unit however he can not add gas to the tank. Is there a flapper valve that has locked or something weird that I do not know about? H E L P

There is a kind of valve in the fuel tank system. However, if your neighbor has been in the habit of topping off the tank after the gas pump clicks off, he may have damaged the charcoal cannister which is part of the emmision control system. Expensive fix. Year and model and mileage?