Gosh, I'd just tell him he's free to use his steel on his own knives, not yours. Why is it more complicated than that?
BTW, I recently bought one of the Big Stick style (1" diameter) ceramic rods. I was initially disappointed to find that it is coarser than my other ceramic. But after using it awhile, I really like it. When an edge starts to go, it gets a swipe or two on this, and then a pass on the finer one to polish. Have you tried a Big Stick?