'Awful' economy and performance could be caused by a host of problems, so your pal needs to be very sure of this diagnosis.
Glazed bores (won't be the pistons, but may be the rings) usually occur on new engines that have been run in too gently. Dosn't normally happen in service. The pistons and rings have developed the usual hard glazed surface before they'd had time to corrrectly mate together. That's why it's now recommended not to change the engine oil too early, but to allow these components to wear together.
It shows as piston blow-by and heavy oil consumption. It's usually cured by removing the pistons and 'glaze busting' (finely honing) the bores, to allow running in to start again.
You've got to be careful about doing this just by removing the head. It may be possible with care to do this manually and keep the debris from the pistons. I've heard of this being done, using emery cloth on the bores when the pistons are down, and it's very easy to leave some of this in the engine, with predictable results.
Perhaps leaving the oil in for a good while and giving it a hard time may also effect a cure?
I don't quite understand this. Cylinder bores (not pistons) do tend to become glazed over time but this shouldn't affect economy and performance because the piston rings will have taken on the same profile as the bores and the gas seal should be excellent.
Has your friend recently had some major work carried out on the engine? If he has had new piston rings fitted without the bores being de-glazed, the new rings will take ages to bed in and this can affect the way the engine runs.
If glazed bores is the problem, the cure is removal of the head and sump and, with the crankshaft either removed or very carefully protected (preferably with adhesive cloth tape around the pins), the bores de-glazed with a special honing tool. On old-style engines, de-glazing could be carried out at home with some emery cloth but modern engines are much more particular about the surface finish.
Decoking won't help at all if the bores are glazed and, as you say, decoking fluids or any other potions fed into the engine will very possibly damage the catalytic converter.
John, you beat me to it. You must be a faster typist than I am!
I'm pleased that our answers more-or-less agree. The only thing I would query is if piston rings become glazed to an extent that can affect engine performance. Also, I would never try and de-glaze with the pistons in place because you can't get to the whole length of the bores and there is every chance of leaving an abrasive compound behind with dire consequences.
I think they agree completely. Yes, I was referring to a somewhat dubious and not-to-be-recomended practice for deglazing bores - perhaps I should have made that clearer.
As for performance, I'd agree. Blow by and oil burning certainly, but probably a limited effect on performance. If this car has these symptoms, plus poor performance, I wonder if it's bore/piston wear or stuck rings actually causing the problem?
My chappie saw what you had all said, then decided to take your advice, not believe the garage, and take a look himself, with the help of his dad. Checked everything, then (by his telling) scoffed as his father said "why not check if there's a spark?". Persuaded at last, he found that at least one of the cylinders was getting no spark.
Back to the diagnosing garage, where they gave him a thingy to look inside the engine (where he could neither see nor understand anything) and told him his pistons were glazed and the bore ridged. He explained about the spark and they laughed at him. He eventually forced them to replace his (and I get vague here) coil box (???) with a spare they had and, hey presto, car runs fine. Buys a replacement coil box (or whatever) for a couple of hundred quid from VW (no-one else had one) and he now says the car is pulling better than at any time since he's owned it - he even thinks it might have been off when he bought it.
Result: Backroom boys 1, Dodgy dealer 0
Other result, he's been going to that garage for a while, and now, thanks to their incompetence, he's told everyone he knows what they are like.