Easy to check return flow from engine
by disconnecting tube from rubber hose going back to tank and placing in a jerry can. Run as usual, making sure that jerry can doesn't overflow. If engine
does not quit then you have found the problem but this is not likely.
Usually, minor air leak in supply side or plugged supply hose but this would normally result in dropping fuel level in glass prefilter (racor), (There is one installed?) and would be readily apparent. Does he have to prime the engine after stall?
Also could be lift
pump, especially if tank is very low to engine.
My guess is dirt in tank blocking pickup and then when suction falls off after engine dies, particles clear allowing fuel to be picked up. This is especially true in rougher waters. Biggest problem with fuel tanks!!!
Clean frequently so that your engine runs when you need it.