Interesting case. The article seems to indicate it was his experience, rather than his certification, that put him in the crosshairs. This article indicates that he was an instructor, and that the charges were eventually dropped: https://www.google.com/amp/s/www.bbc.com/news/amp/uk-england-sussex-35350295Stephen Martin was charged with homicide when his girlfriend died, probably from immersion pulmonary edema: Scuba enthusiast fights extradition to Malta over dive deaths
It seems like you'd need a perfect storm of extremely unlikely events for this to be an issue. How many of us have been on a dive where someone was killed or injured badly enough to sue (or for the state to bring criminal charges)? How often does the injured diver/victim's family/state go after the buddy, or other amateurs on the dive, as opposed to only going after the pros involved? What are the odds that, if they did, they would drop the suit upon finding out you were only AOW (and wouldn't find out that you'd done the rescue course but declined the cert, or that you'd read the book and worked with an instructor informally, or that they wouldn't try to use that against you), whereas they'd pursue you to the ends of the earth if you had taken Rescue? And all of this, of course, is assuming the Rescue course didn't make the difference in the first place in allowing you to save them.
Also, I'm way out of my depth here so don't take any of this for anything even approaching advice, but... in the U.S., there is generally no duty to rescue absent certain circumstances, such as a special relationship to the victim (haven't yet found any authority on whether scuba buddies count for this; they might). But another such circumstance that can confer a duty to rescue is if you initiate and then abandon a rescue, the logic being that you might have preempted someone else who could have rescued the person. If Rescue teaches you your limits and keeps you from starting something you can't finish, might it actually give you some legal protection?