Isn't it a bit funny how most shotguns and rifles/carbines are equipped with external mechanical safeties, and yet a lot of folks cry 'foul' and look aghast at someone suggesting they use a semiauto pistol equipped with an external mechanical safety?
The presence and proper use of external safeties ... whether required by issuing agency mandate/policies or just personal preference ... seems to require reasonable familiarity, along with necessary, frequent and consistent proper training & practice. Proper training helps reinforce proper skills ...
If your agency or personal preference is to carry a TDA pistol with the safety in the ON SAFE position, then you'd probably better invest in some frequent and proper training to instill the necessary skills to disengage it under any and all anticipated situations ... particularly stressful, life-threatening ones.
Also, don't take this wrong, or intended to be overly critical ...
But while you didn't mention any agency policies or personal preferences being involved in this "exercise" ... if
Dan routinely carries his pistol in the OFF SAFE/READY TO FIRE position, and you placed the pistol's safety in the ON SAFE position before handing it to him, because of your preference or own agency requirements, then either his trust in you is misplaced when it comes to trusting you in handling his weapons, (and without at least checking and confirming the condition of the weapon when you handed it to him?); you
assumed he carried his pistol in similar condition to yours; or, you
presumed to inflict your preferences and handling methods on him, and in a 'dangerous situation, at that. Any lessons to be learned?
If, on the other hand, Dan routinely carries his pistol in the OFF SAFE condition, then perhaps at least one of his training priorities just received a bit of a wake-up call. Removing/disengaging a manual safety under stress requires dedicated training, performed in as frequent & as consistent a manner as necessary for the skill to be 'available' and properly demonstrable. He has only himself to 'blame' for not checking and confirming his pistol was in the normal condition he expected and trained for it to be ... even after you handed it to him. Distraction can cause serious issues to occur, even within the safe confines of the 'prep van'. Training, training, training ... properly.
Properly, safely, consistently and smoothly depressing the safety lock (thumb safety) on a 1911-type SA pistol requires a skill set not needed on many other handgun designs, such as a DAO-type pistols, and that's why some agencies and individuals won't use them. However, TDA pistols carried Ready-to-Fire, equipped without spring-loaded, decock-only mechanisms, commonly require a 2-step manual action to decock and return the pistol to the Ready-to-Fire condition ... and training can help instill the physical skill ...
Less than properly reasonable & necessary familiarity with any given handgun's operating requirement, though ... gained by knowledge and experience ... maintained via proper, consistent and frequent training, preferably prepared and conducted to instill and enhance the necessary proper skills under circumstances safely devised to closely simulate the actual anticipated circumstances (as closely as prudence and safety considerations permit, of course) ... is something to truly consider. It's also a mouthful ...
I see different cops & CCW folks forget to disengage manual safeties quite a bit. Sometimes it's a case where they forgot to complete whatever 'decocking' manipulation was required for their particular pistol so it could be in the Ready-to-Fire condition, and other times it's seemingly because they just weren't reasonably trained enough to 'remember' disengaging the safety was necessary before the pistol would fire. The scary times are when the folks don't consider it a problem for their pistol not being able to be fired when they need it to fire, and argue that the same thing would 'never' happen for real, and don't want to hear any helpful training or remediation ...
Other disappointing instances are when some folks depress their magazine catch, intending to decock their pistol ... and become annoyed when their magazine falls free. Talk about lack of maintaining proper familiarity with a defensive weapon. I'd be something other than annoyed, and immediately seek, or at least welcome, some remedial training.
If proper familiarity with operating a pistol disappears under the minimal 'stress' of normal range conditions, or the elevated stressful conditions of FATS, Simunitions or Force-on-Force training exercises ... I'd worry about it being suddenly available under actual life-threatening conditions. Some folks just seem to want to get it over with and get away from the range, though, and I'm remembering both cops and CCW licensees, too.
Lack of proper familiarity and awareness, and the necessary skills in using a manual safety, is also an issue to consider when a cop decides to carry a personally owned off-duty weapon, which operates differently than his/her issued handgun, on his/her own time. Some folks seemingly just don't want to hear about it, though ...