I'm not sure, but I found an empty painter shell in the woods once. I picked it up and it started falling apart. So something has to be done.

However if he just wants the bone, I suppose he could just wait until it was decomposed. Then soak it for a several minutes in hygrogen peroxide. I have a boxie shell that I found in the woods. It is just bone and it keeps just fine.
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0.1.0 ornate box turtle
2.1.0 eastern box turtles
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.9.0 chickens
1.0.0 Dutch rabbit
3.2.0 adult children
1.0.0 husband AND
0.1.0 grandbaby!!