I had a friend over for the holidays. She left her snow boots out. I should have put them up when I saw them, but I forgot

So Spirit, 9 months, chewed one a bit. My friend said I needed to teach him boundaries, what's off limits.
My take on that is I know what is tempting for him to get into and I should be proactive with making sure things are picked up so he can't get into them.
How do you teach a young dog that chewing boots is off limits? Teaching boundaries .... like teaching what's yours is yours and what's mine is mine and you can't cross this boundary.
Seems absurd to me, but am I missing something?