I'm not saying that it's the greatest thing to do, I personally wouldn't as I don't see a need for it, and as stated, you hardly notice the thing because it sits flush with the quarter panel.
However, with that being said, if he realizes the safety concern attached to the modification and accepts them, then all the power to him. It is in the end, his car.
Personally, IF and IF I were going to do so. I would install a one way valve from where the filler tube connects to the tank to prevent any gas from spilling out if an accident were to happen. I know that in some of the later year Caprices (my '95 and '96 both had them), GM had installed such a device as a countermeasure just incase you were rear ended so no gas would come spilling out. They also made the tube in such a way, that if you were rear ended, the tube would break on one of the bends as opposed to where it connected to the tank.
Just saying.