I'm not so sure this was what happened. The fact that the valve was nearly off makes me think that she or one of her buddies set the gear up, turned the tank on and checked pressure, checked the regs and the inflator. Then they turned off the tank to prevent any small loss of pressure while they waited to start the dive. Only problem is, no one remembered to turn the air back on and when she checked her SPG she had pressure and probably got a few breaths of the reg to boot.
All speculation, but I would not be the least bit surprised if that is how it transpired. I've seen it done.
I agree. It's very possible, and, I think, a very dangerous mindset.
Once, many years ago, a similar thing happened to me. I was on a boat on vacation in St. Thomas. Had all my gear set up early. Went forward to use the head (unbeknownst to me at this time the divemaster turned off my tank "so it wouldn't leak". 15 minutes later we arrive at the site, I slip into my rig, double check pressure (2800psi) and do a giant stride. In the water all is good, I drain the last of the air from my BC and descend. At about 15 feet I'm suddenly out of air. I check my guage-zero. I swim up, inflate the BC with a couple of breaths and swim back to the boat. When the DM asks me what's wrong, I tell him I've got no air, and he says "oh yeah, I shut your tank off so it wouldn't leak" reaches over my shoulder and turns it back on.
My pre-dive routine has always been:
1. Assemble life support
2. turn on gas
3. check function of regs
4. get dressed
5. double check
6. splash
I knew I had turned on my air- it never occurred to me that someone else would shut me off. I double checked pressure (I now take a couple breaths while checking pressure to make sure pressure is stable (and make sure no idiots tried to help me)). I am now extremely leary (violently defensive) about anyone else handling my gear.
Needless to say that divemaster did not get a tip from me that day (unless you count my friendly advice to keep his hands off other peoples gear unless asked) but, in retrospect, he did make me a better diver.