Here is the link to a short video I shot showing a diver in distress during a 120' deep dive in Cozumel.
First he needs his BCD adjusted. Then he needs more weight. Then, apparently, he thinks he is low or out of air even though his reg free flows when he takes it out of his mouth (0:22 mark) and air flows when he purges his hanging reg (1:56 mark). I did not observe him try his own octo alternate air and cannot say whether he did or did not try it before sharing air with the DM. Although you cannot see his face clearly in the video, I could see it and in my opinion he looked very frightened and panicked, not calm and cool.
DM provides octo and the two share air for the most of the rest of the dive, 15 - 20 minutes. I was surprised DM did not end the dive.
After the dive the distressed diver told me he bought his gear used from a scubaboard posting. I'm under the impression this was the first time he had used the equipment. His extremely long hoses (extra length wrapped around his neck) suggests the seller was a cave diver. He didn't speak to what happened with the sharing of air and I didn't press him on it ... in the end everything turned out OK and I didn't think it my place to interrogate him about the incident.
YouTube - Things Go Wrong SCUBA Diving at 100 Feet