Besides the obvious human error on the part of the ooa diver, the thing I  think is significant is that this turned out fine because no one  panicked.  Equipment failures and human mistakes are common enough.  I  think how we deal with them often makes a difference in the outcome.
I suspect  the diver checked his guage and saw it was at or near zero and sought help, but had not  yet reached the point where he could not draw a breath, or the  situation may have been a bit more dramatic.  I assume after he was on yours or the DM's air, someone checked his tank valve before the ascent?