I had him try it this morning and he can only get the lock disengaged using his thumb - which means his fingers are in the way of the blade.

However, quite frankly, that's how I was working the mechanism last night trying to loosen it up too. I can do it now with my index finger without anything getting in the way of the blade, but not near as easily. It actually physically hurts a little bit to apply enough pressure to get the thing unlocked.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen