I think when they're bored and their chewing habits aren't directed to things they can chew on, they'll chew anything including metal, rocks, etc. When I'm training a puppy at home, my husband I keep a small rawhide knuckle bone in our pocket, so it's handy. As soon as we catch them chewing on something bad, we calmly but firmly say "No", and then exchange it with the knuckle bone. When they lose interest in the bone and leave it on the floor, we pick it up, pocket it and start over until training is complete.
My dog didn't chew stuff either, except when he was a really small baby he would chew our hands and stuff but never really items. I think it was in part due to my elderly dog's instruction, because he didnt really have a mom he got taken away at a really young age.
So my doggy rasmus took care of him and raised him. But as far as training, I'm not sure. But that is a really good tip, alpha1 I'll be sure to remember that if my friends ever run into this problem