I've changed fuel filters on gasoline vehicles before. No big deal if you follow some basic precautions --
(1) Depressurize the fuel system with the nipple on the fuel rail
(2) Work outside, ideally on a windy day. Was your friend working in the garage or in front of it?
(3) No 120V tools -- battery-powered flashlight
(4) Keep a fire extinguisher rated for fuel fires at hand. And maybe a garden hose for anything that's not fuel that ignites.
(5) Work with the fuel tank nearly empty to minimize siphon effect, and keep unpainted (gasoline dissolves yellow pencil-paint and makes a mess, ask me how I know) pencils or similar handy to plug the lines when disconnecting the fuel filters.
(6) Wear gloves. Fuel spilled on my skin gives me migraines

Sorry for your friend's disaster and glad he didn't lose the house, but this should not be a deterrent to doing your own work on the fuel system.