Used or not, the issue at hand is why this pistol has discharged in this manner twice.
There are two possible conditions which could cause the weapon to fire:
1. Operator action, or combination of actions.
2. Mechanical defect.
For the sake of argument, let's assume that the operator made no errors in handling, especially since this event happend twice, the second time when he was being doubly careful to avoid a Negligent Discharge. (However, the OP should STILL carefully evaluate his actions to verify this.)
This leaves a mechanical problem.
It is possible that there is a cleanliness issue related to the firing pin. However, I should think that this kind of defect might also lead to misfires as well, since any amount of fouling that would cause the firing pin to stick forward firmly enough to impact a primer when chambering a round with enough force to set it off. Easy enough to investigate and easy enough to clean.
If the hammer is, in fact, dropping when it shouldn't when the slide moves forward, then this might indicate either excessive wear or damage to the internals of the related mechanisms. I'm not familiar enough to name the components and what areas to check, but this, too can be checked by a complete breakdown.
All that said...ANY TIME A GUN MISFIRES and you don't think it was operator action, THE GUN NEEDS TO BE INSPECTED AND REPAIRED. If the owner doesn't feel competent enough to do this, then he should take it to someone who is qualified to do so. It should only cost about the price of a box or two of ammo to have it inspected, well worth the money.