That is true but at this point, we don't know if she had a safe, or what kind of safe it might have been. Even having a safe when an unstable person is living in the house present should not be considered secure storage.
A 3-digit combination lock where each digit can be 0-50 has 51*51*51 = 132,651 possible combinations. Using a brute force cracking approach, assume 5 seconds to dial a given 3-digit combination. Worst case will require 184.24 hours to figure out the combination.
At 4 hours per day, he'll have the combination in less than 2 months.
Basically, if you have an unstable person in your home who by virtue of his obsession with video games can focus for extended periods on a task, you cannot trust a safe with a random 3-digit code where each digit can be between 0-50.
When you have guns you have a responsibility to keep them out of the hands of psychos.
Lame excuses do not absolve one of this responsibility.