I agree no one wants to pay out, but out of the things listed only the wing is really something to get really miffed about. Carbon build up causing lumpy idle, not the end of the world, obviously depends how lumpy but I bet it still drives fine, same with thermostat, will still drive ok. Get the two done at once. DV, check before you replace, easy to do, cam follower is pretty much a one off thing and costs £35. I'm. Not saying don't get these things done, but if you cant afford them in one hit the car still drives fine ?. I got clutch slip soon after buying my car, that's want ideal !
If he had said the gearbox went, then the head gasket blew, then yes, totally unexpected and clearly the car is not in great health. But even the very best 10 year old GTIs will need a follower, carbon clean, maybe a DV, and prob a thermostat.
I am not saying he shouldn't be fed up about paying out, but his car doesn't seem at all like a bad one, normal issues tbh. He won't have to do them again for years, if ever !