It's practical advice, but is your S going to feel like you don't trust HIM? Why not just lock up the silver or whatever while you're gone, maybe set up a webcam covering the entry points so you can see if she shows up when she shouldn't, and call it a day?
If she has the right to enter your home, then key or no key, she has the right to enter the home (i.e. it would be lawful for her to break a window to enter). If she DOESN'T have the right to enter your home, then using your son's key would be a crime.
