EDIT: In general, could they replace it themselves? Would it be a difficult job?
This depends a bit on what model the vehicle is. On some vehicles, the valve and the tube from the tank form a complete assembly that is easy to access. On others, the tank would need to be dropped to gain access to its top (where the valve tube is connected).
If necessary, dropping a fuel tank is an involved process. You would need a vehicle hoist for the car, and a vertical jack to hold the tank while it is being lowered. Best practice would be to ensure the tank itself is empty or as near empty as possible, to reduce weight and associated risk.
Rusted connections can also be an issue. Beside needing to actually unfasten the old valve tube, you would need to take care and make sure no old mud or bits of rust fall into the tank and pollute the fuel.
In general, the old separated ladder chassis found on some 4x4 grants easier access and perhaps can make tank removal avoidable. Modern unibody car construction makes things more complex.