If the other pilot was in the cockpit monitoring the parachutists, how could he/she not notice the deceased pilot unstrapping and opening the door?
Well, unstrapping doesn't need to be that obvious and I assume the co-pilot was twisted around in his seat looking into the cabin, not paying attention to his colleague. Once unstrapped it would only take seconds to open the door and jump. Seems pretty straight forward to me if it was deliberate. Remember, a Norwegian passenger, being medevaced, managed to jump out of an EC225 window a couple of years ago and that was with two colleagues monitoring him!