The old Brison company noted 70 or 80-1 synthetic ratios in the manual. If he's been running it 4 or 5 years it's an old Brison like the ones I have. If it's been years since the needles were checked it's time they were looked at. Tell him to quit being so lazy and pull the cowl off once in awhile
You never know what's been living in there if you don't[X(]
All of the engines we use benefit from a warm up prior to tuing or going for the blast off after starting. Mixture settings change as the engine expands during warm up, and idle rpm will be higher after warm up than immediately after starting. Engines with leaner oil mixes warm up a lot faster, which should be a warning indicator all by itself. Dependable mixture settings and idles don't occur until ater the engine reaches cht's between 80-90c, or 176-194f. There goes the theory that engines running at cht's of 200f and below are healthier...
Tune them to run slightly rich of peak rpm. Maybe 100 or so below peak, and all should go well. Flying tells all of course.
As for 500 hours between rebuilds or worn out, that's totally dependant on how the engines are run and treated. I've seen some that didn't make it 30 hours before they had to be torn down, with an average t.b.o. before a top o/h to be in the area of 120 to 200 hours . Those running the snot out of them will have shorter times between over hauls, especially those that heat them up and keep them hot flying lots of hovering and torque rolling 3D. The circle flyers, like warbirds, can expect a lot more than 500 hours of total engine life if they started with one of the better engines. Then again, the first nose in impact sends the engines back for parts and labor anyway, and that usually happens long before they are worn out
Regarding the melted ring lands. I haven't seen that very much in air cooled "flying" 2 strokes. When it does happen the piston wall usually has transferred a lot of aluminum to the cylinder wall as well, indicating that the owner ran it so lean or low on oil the engine was melting. Rarely will it be from poor cooling inlet air. With some of the engine types you noted, in many cases the pistons have secondary compression rings and oil rings. They run drier and hotter at the tops. Perhaps due to over head valves and such. For our stuff we get excited when we have a two ring piston. Those are the best for heat transfer and maintaining compression as they heat up, but like anything else can get toasted by poor operational methods too.
The ring materials that are used in our model engines are not anywhere near the quality of automotive stuff. At least one manufacturer uses two different qualities of rings in their engines. One type is absolutely terrible and unfortunatey the buyer will never know which one they are getting in an engine when it's shipped.