DE0000023 should be the correct tire if I read the chart correctly.
What radius track is he running it on? How is the track, is it new, old, fastened down, overall condition, aligned properly? Is this happening in one area of the layout?
I had a rash of tires come off lately, but I feel it was mostly from age of the tires. I do have a Weaver 4-6-0 that just threw a tire, but I have no way to determine what caused it. I haven't replaced it yet because I plan on doing some updates to the engine and I'll do it when I do the updates.
I use 2 small flat head screwdrivers and a coupler of metal clamps when I put on new tires. The clamps keep the tire from coming off and the screwdrivers are used to go around the wheel as I put on the tire. It's that or I need to grow a 3rd hand.
I've replaced a few tires where the diameter of the new tire was good, but the width of the tire was too wide. I install the tire then take a new single-edged razorblade and, with the engine upside down, trim the width while it's running. So far it has worked well.