I have allergies to metal, too. It's the nickel used in most metals that does it to me and includes being allergic to sterling silver. I can usually do 10K gold and more. Most hardware has nickel in it and all the silver coloured stuff has nickel coating, so it's bad for me. I've found that copper is OK for me, so your customer might try that, but when the clearcoat on most copper wears off, it'll make your skin green or black where it rubs. Clear nail polish works, too, but it wears off eventually and has to be reapplied. You can spray some items with a clear acrylic coating - try Krylon indoor/outdoor 'cause it comes in matte or gloss and is flexible - but it will work its way off, too. I would suggest that if you do this, you tell your customer and don't make any promises. One customer of mine told me she puts fabric bandages (not the pad) on the back of the rivets on her jeans pockets and metal buttons - it stays on in the wash.