I don't know guys.... It really could be ok depending on how he sharpens... I have knives I've thinned at least once a week and they are still around after almost 8 years. Just because you're thinning doesn't mean you have to be removing a ton of metal... It could be just a bit each time... Just enough to keep the knife right where you want it geometry-wise
Is there something particular you look for when you can say that you've remove enough metal when thinning? Is there going to be a burr too?
i just eyeball it... a large part of being good at sharpening is having experience using the knives (or getting great feedback from someone who uses them), so you know what will work for a particular steel, HT, intended use, etc. Thinning doesnt always have to create a burr (and in many cases doesnt), but it is possible to thin so much as to create a burr. It just depends on what you are trying to do and how thin you want to go.
+2 for the diagram
Assuming you are not screwing up the geometry by thinning, how does repeated thinning reduce the life of the knife? Rarely ever are you hitting the edge while thinning, the knife isn't going to get shorter. It will get shorter from sharpening, but not thinning. It will get just get skinnier. When you go on a diet, you lose weight, you don't get shorter.
"God sends meat and the devil sends cooks." - Thomas Deloney
Mostly the knives that would be affected are truly even flat grinds.