It’s easy to gain access to a safe if you live in the same house and it gets opened regularly.
A tiny camera or fingerprint dust could easily be used to find the code.
Or, if the safe doesn’t have a lockout feature someone could simply try every combination for several days while the mom isn’t home.
My point is that she failed in her responsibility to keep her firearms out of the hands of a son she knew to be psycho.
Do you propose she should have moved her guns off the premesis? Or just not bothered with any security measures at all - becuase hypothetically any security measure can be eventually overcome?