Good that someone took it home. No different than finding a seashell. And the museum types will fawn over it, catalogue it, and put it in some drawer in a non-public area storeroom, and hide it away forever.
Now someone gets to enjoy it.
Taking fossils home is usually theft unless you own the property. It’s certainly theft under public land use laws.
If this wasn’t theft it’s a sad loss for getting more knowledge about mastodons.
The owner could gift the tooth to a museum for a couple of years like people do with art works out of private collections. Get it dated and get DNA and what other tests can be done in that time lets everyone in on what secrets it holds.