Judging from your description it could be many things but assuming it's a fuel problem i would simply have the Fuel pressure measured.
Once the engine starts it should give you a value which (exact value is not really that important as long as the engine runs), then rev up the engine and if the value then dramaticly drops it's a fuelpump/filter problem !
He's not gonna be too happy if that pump fails since it will require him to lower the entire rear axle to lower the tank and acces from the top
Or he could cut a square 'hole' in the rear trunk area to acces the tank directly (some people prefer to do that as it saves quite a bit of time.
But let's not jump to conclusions just yet, i would first have him go to for instance Opel Haus and have the fuel pressure checked, especially under load.
