Deep bounce dives on a single cylinder

I am recalling a story told to me by a friend whom I have dived with many times, she is a very competent recreational diver. She has not been trained for deep or decompression diving, she is a pretty good divemaster by all accounts, I have seen her in action on many occasions and can attest to her diving ability in a recreational setting. So where did it all go wrong?

First it started with trust, her buddy was an instructor with 5000+ dives experience who had worked in Thailand and Turkey among other destinations, that is high profile European and South East Asian destinations. With that many dives in high traffic dive resort destinations, he could be considered a full-time professional diver.

The dive plan was vague, she was to follow him down the line to an unspecified depth and follow computer decompression requirements back to the surface. Their equipment; standard recreational scuba arrangement - single 11 litre aluminum cylinder, standard K-valve, jacket type BCD, a single wrist mounted dive computer and standard recreational regulator with alternate air source.

The conditions - strong current, no surface wave chop, sunny day and a light wind blowing. She followed him down the line, he was descending quickly and she was encountering difficulty keeping up to that rate of descent. She was able to grab him at 40 metres, signalling to him that everything is not ok; he signaled back that everything was ok and continued the descent at the same rate.

She encountered severe nitrogen narcosis as the descent continued and stopped, at this point she decided to ascend alone, depth unknown as she was unable to read or comprehend her dive computer display instead using her exhaled bubbles as a guide. Apparently, the ascent rate alarm did not sound during her blue-water ascent. Her instructor continued his descent unaware of her difficulties nor did he seem to care about the person he was diving with.

After some time, her buddy did notice her missing and began his ascent, he had gained a decompression obligation greater than her due to his greater depth and longer exposure at depth. He did not manage to catch up up with her during her (and his) hang time. Due to her distress and anxiety, she had consumed a fair amount of gas in her single cylinder and was running dangerously low with still time remaining on her shallow 3 metre decompression stop.

There was no decompression hang tank in the water, but what did it matter anyway? She didn't make it back underneath the boat at any rate. She had to perform her in-water stops without the aid of a lift bag and reel as she did not own such equipment instead trying to maintain depth without any aids. Hence, the maintenance of depth was pretty poor with notable depth changes during the decompression phase.

Upon arriving at the surface, she wanted to know what the hell happened, she was not experienced at that depth and the nitrogen narcosis was almost debilitating. She had plenty of time to think about it during her surface swim back to the dive boat, she was pretty upset. When he arrived back at the boat some time later, his attitude was nonchalant, this angered her as she felt real fear for her well-being.

As she had consumed a significant amount of gas at depth, had she not turned and ascended, she would have emptied her cylinder at depth, not just on the hang. As no turn pressures had been discussed let alone rule of thirds, it was just luck she began her descent when she did. Naturally, they had not done any gas planning, nor contingency planning for that matter. The wash up - at what point did this do go wrong? How could have things been different?