Help needed with sheath tarnish

i have finally convinced a friend of mine to pick up the hobby of knifemaking. (It's kinda like offering a friend free crack). Lol! He has his first knife and sheath made and did a really nice job on both. He bought his veg tanned leather at Tandy's and it tarnished his blade VERY quickly while in the sheath. The sheath is wet formed (and dry) and I was wondering if there is anything you guys would recommend to help with this problem? Something that won't soften the leather too much? Thanks gang!

I wax and polish my blades before putting them in my sheath, and never store a knife for long periods of time in the sheath. I also seal the insides of them with Tan-Kote. Just pour it in, move it around, and pour what's left out. Let it dry over night.

I use Tandy leather. When all work is finished on a sheath or holster, and it is thoroughly dry, I give it a heavy dose of mineral oil, then let it drain and sit for a few days allowing the oil to equalize through out. I never get rust, even if the knife, or gun stays in the leather 24-7, which my .44 Ruger has for seven years now.