As long as you both enjoyed it--- :54:
This is a very common occurance, and as you noticed, not the end of the world.
If he was really that oblivious, maybe not bother with touching him- it might have gone from harmless to awful very quickly. It's would be pretty harmless until he surfaces, at which point the mouthpiece would be tugged by the hose. Still not life threatening.
Sometimes it's best to just watch and be ready. With your observation and SCUBA skills, I would suggest quietly mentioning to the DM that he might want to keep his eye on this diver. Sounds like the DM had enough work, though.
To assist a more aware diver, he should be signalled from directly in front with a "wait here" gesture (both palms down, with a down up motion). Then go behind the diver and using your knees to brace the bottle, slide it in the strap and cam it closed.
There are fellow divers that are a danger to themselves, there are some that are of danger to others- it is our own responsibilty to make sure that they do not endanger us. To be a good samaritan, you gotta stay alive, first.