question? the nut only comes into play on a open string Right?
Next Segovias sound comes from his fingers , nails and atact!
I tried testing the wear of diferent miterals by putting each into a vice and take a string and try to saw the nut into.
Do the same pressure and the same number of strokes and see the wear of each.
If the issue is that the customer is convinced that Black Buffalo Horn is going to give him the sound he wants, you are likely to be fighting a losing battle to convince him otherwise. And it may not be a kindness to try to convince him otherwise. For example, if the player got the advice to use BBH from someone he respects (perhaps even youtube), you will be standing between the customer and his advisor, and that's not a good place to be. If the customer goes with your advice and choses another material, he will take flack from his advisor. If he goes with his advisor, then is unhappy with the result, he is likely to feel sheepish about not listening to you. OTOH, if the customer asks for your advice, give it freely - then it's really his decision.
If the issue is color, go for a black material. Or even dye bone black. I've done that a lot for customers who really want a black nut.