The Meaning of Life, the Universe, and Everything.
Tell you what: I'll help you if you help me.
I know the solution to your problem, but I have one of my own.
See, I can't get the skins on my server to display to other players. It's infuriating. All they get is a warped version of the "Untextured" texture.
I tried adding the skins to the same folder on the serverside, but to no avail.
Why don't you just tell him how to do it? Also I think the problem is because the other players don't have the custom skins you have installed to their computer. The Untextured texture only appears if a textures is missing.