They didn't replace the
ECU, they replaced the
PCM (Power Control Module).
I suggested to him maybe injectors, and possibly do the oxygen sensor, but the fact that lpg consumption is about right, I doubt it's that.
I asked him about how often he runs petrol, and he said he does about 40km of petrol running after every tank of lpg. At least that should eliminate that side of things.
I'll get him to run some injector cleaner through a few tanks of petrol, and change the fuel filter, then see what happens.
Thanks guys. I'll report back with results if any.