Ok, he’s referring to the knee CUP, which I have personally never seen in anything OTHER THAN plastic (or better yet, heavily padded foam and plastic)
But just to be clear, my primary experience with stilts and instructions for safe use of them is in the context of stilt walking in parades and circuses.
Is you friend asking about drywall stilts ? There may be some kind of subtle difference of which I’m unaware, but general principles are still the same.
Most stilts are a rather bare bones type of arrangement. The knee cup (if there is one) is not what primarily provides the support. It is the STRAPS UNDERNEATH the knee area as well as the ones across the shin area which are attached to the side framework which provide the support. If there is any kind of failure in that area of support, it doesn’t matter whether there is a knee cup or not.
It is critical that they be of sufficient strength to not suddenly snap with the strain. It is also important that they be tightened sufficiently and most have Velcro to provide the tightest fit.
What most performers prefer is stilts without a knee cup at all. They just go to a sports store to get their own preferred knee pads (like skateboarders use).
This provides much more protection and comfort than just a little hard plastic kneecup can do. And it’s much better in case of a fall.
If it were me, even if drywalling, that’s what I would do. Your friend can do as he likes. But if he already has knee problems, I think he’d do better with knee pads. Drywallers are not immune from falls either :)
Most big home improvement stores like Home Depor carry drywall stilts bit with and without knee cups so they aren’t that difficult to find. Or searching online brings up lots more options. Personally speaking, I would want to see something like this and try it out in the store before buying. They aren’t cheap.