It's probably time we reviewed the topic of this thread:
"What if you need to use some of that 500 psi contigency reserve?"
This is not about "ownership" of breathing gas or whether or not one would or should "share air". The question is not about proper gas planning. The question is not about responsible diving practices.
The question arises, in my opinion, from the current train of thought that a diver should surface with no less than 500 psi of breathing gas remaining in her/his tank/s.
The basis for this can be traced to such reasons as gauges being incorrect, depleted tanks will corrode, and so forth.
The answer to the question is so obvious that it makes me wonder why it is even asked.
If you need to breathe and there is gas in your tank . . . then for God's sake take a breath.
Nuff said . . .
the K