Very sad if this is indeed the case, but there are still several big questions . 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? If in the cabin, it is surprising that the deceased pilot managed to unstrap and climb out of a snug cockpit without disturbing the flying controls. And if it was a jumpmaster in the back rather than a co-pilot, then he/she did rather well to land the aircraft safely afterwards.

As already pointed out, it is virtually impossible to move from the cabin to the cockpit (or vice versa) of an Agusta A109. It is a very small cockpit and ideas such as parachute for the pilots, etc, are similarly untenable as there simply isn't enough room.

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!

What would you be able to do about it?

I think it's probably a good thing for all concerned that the other pilot did not notice the event. It's unlikely that they would have been able to prevent it happening if the individual was determined and any struggle could have jeopardised the aircraft further. Plus. At least the surviving pilot is spared the memory of watching the event happening although seeing the empty seat will have been bad enough.

Not that it's directly relevant, but it's not a FAR. What drives the use of pilot emergency rigs are either a) they are required by the skydiving door modification STC, or b) common sense. Also, they are usually purpose built rigs identical to those worn by aerobatic pilots and passengers (which is required by the FARs), not mil. surplus.

Here's a good reference for budding aerial delivery drivers: DiverDriver.com

Like I said, it was a long time ago. The fancy square rigs were pretty new and out of my price range at the time, so most of us were jumping modified military surplus stuff.

Nothing at all other than focus on flying the aircraft safely. The remaining pilot managed it anyway, and kudos for achieving that despite a horrific shock, but the less time the only pilot spends looking backwards, the better. As others have said though, it is probably slightly less unpleasant for the survivor not to have seen his colleague go.

This is all very odd. The pilot would have known that the parachutists would have exited quite quickly, but that would have still left the jumpmaster and/or co-pilot in the helicopter, who would potentially not survive with the alleged departure of the pilot.

On the brighter side
 

This incident happened back in 1987, didn't involve any passengers and ended happily. Again kudos to the copilot for not freaking out. As An ex sport jumper I agree with all that has been said.
www.nytimes.com/1987/09/04/us/pilot-is-survivor-in-freakish-mishap.html