If he broke into and stole the guns from a locked safe he did not have the combination for, then the parents should not be held liable. However, if A. he was given the combination/access or B. the guns were accessible outside of said safe then they should be liable.

If you feel that your child/teenager/young adult is responsible enough to have access to or some of your weapons and then does something irresponsible then yes, you should be liable.

I agree with the Sheriff though, society has changed.

This amounts to victim-blaming, because the parents would be victims of theft. They didn't pull the trigger. Manslaughter charges would be completely inappropriate.