https://www.nortonabrasives.com/en-us/product/crystolon-sc-pocket-stoneI only see a one-layer crystolon of the right size, unfortunately, two-layer ones are available only in larger sizes.
https://www.sourceatlantic.ca/Product/104235https://www.turnersupply.com/Product/805287600Alternatively, he can use alumina ceramics, leave one side finely ground, grind the other side with silicon carbide powder F230. I bought a fine ruby ceramic stone (
https://www.ottofrei.com/Degussit-Ruby-Bench-Stone-4-x-1-x-3-8-Loose - Al2O3 + Cr2O3 for ruby color), one side of the factory processing, the other side I processed with SiC powder for more rough work. This is enough for easy sharpening in the field - remove damage with the coarse side, finish with the thin side, if required.