Break them on purpose and then leave it to his better mechanic to finish off.

I've been in this situation with spark plugs on the ohv fords, in fact we've got a KA in now which has a misfire on cylinder 2 but the plugs are so rusty we tried to explain to customer but with them being Chinese its almost impossible.

If it was me i'd either remove the glow plugs you've fitted and not charge him for your time/effort you've put in or just walk away and take a loss because i think its a job the customer (from how you've described it) isn't going to pay proper money for anyway.

I wonder how much money over a year us mechanics mobile or workshop based lose.