My buddy has had an out of air experience - we were diving in Hawaii and his air consumption rate was better than normal, but not so good as to be outside the realm of possibility.
He was around 600 psi (plan was to head up at 500) and we were around 45 or so feet - he was getting ready to head up when the DM pointed out some sharks.
Being 100 higher than the minimum to head up, he hung around for a few minutes to watch the sharks.
He had just barely started his safety stop when no more air - gauge said 500 but nothing was there. He got the attention of the nearest diver (also doing a safety stop) and they shared air for an abbreviated stop before surfacing.
He didn’t try to get my attention, despite being my buddy, because he didn’t want me to panic (as I have a lot less experience) and as I was still checking out the sharks.
I wasn’t worrying about him because the two of us, while buddies, dive more or less as solo the majority of the dive - before he started his stop he had let me know what he was doing and that I should continue with the sharks and that he was fine on his own.
Lessons learned - even if you watch your gauge, things sometimes malfunction. We had been using rental gear and after this we won’t rent regs and gauges anymore and our own gear is regularly checked and serviced.
I also reminded him that I consume air far more slowly than he does (SAC is between 0.2 and 0.3 in warm water) - he runs out of air long before the NDL limit, I often have to surface due to NDL or because the DM wants to go.
We spent extra time reviewing each other’s gear so we both can find the alternate air source without assistance of the owner.
I have started providing a “buffer” in my own air reporting - for example I’d report 2,000 PSI when I have 2,500 so if an emergency happens I know I have enough to get us both safely to the surface (and I modify that buffer as necessary based on the dive conditions).
I check in with him more often because this has caused anxiety issues for him that he is working through.
If I don’t surface with him, I watch him while he heads up and keep visual contact as much as possible until he’s on the surface and boarding the dive boat. If visual contact isn’t possible, I surface with him regardless of how much air remains in my tank.
If he can’t get the attention of the DM, I burn a bit of air and establish physical contact with the DM and explain what is going on - if the DM ignores the info, I hear back to my buddy, who is on his safety stop, and send up my own DSMB. If it’s more urgent I send my DSMB immediately and let it beat us to the surface.
We’ve also established ways of communicating to each other what we need in regards to going up - for example, 1 thumb means “I am going up but you don’t have to come” - at which point I ask if he’s okay. If so, he heads up and I keep an eye on him until he’s safe.
If it’s a two thumbs up, we both go up and I know we may blow the safety stop.
If it’s a one thumb and not okay (even “so so”) we both go up and communicate about the issue so we can surface as safely as possible.
so far we haven’t had to do a two thumb dive but it helps him knowing we are prepared in case of emergencies.
The decision to keep diving or quit is entirely personal but I would suggest trying to logically think through the scenario and factors that led to your situation and how you could address such things in the future. At that point I’d make my decision as to how to proceed (quit, continue under more benign conditions, continue but take more training or find a diver to be a mentor, etc).
But **** happens, even to the best divers - all you can do is decide how much risk is too much risk for you.