I had previously posted this but I'll post it again: He 100% was not a solo diver but I take your point. I see, from re-reading my original post, where you could think he was a solo diver as I did use that term. I should have said "the other diver was by himself and was the DM's buddy" or something along those lines vs 'solo diver'.How do you know his experience level? Did he tell you?
Not seeing someone check their gauge means little. If someone has AI (Air Integration) or is fairly experienced, you may not notice them checking their gauges, because it can be done with a glance. He didn't run OOA, so it's almost a little off-topic.
If they were buddies, there's not much you can do about someone who intentionally swims off. Given this was a drift-dive, and assuming he wasn't actually solo but rather buddied with the DM, that looks a lot more like a "team" dive.
A more challenging question would be, what should the DM do? The DM chose to stick with you and your wife when the other guy swam off to look at sharks. The DM may have been buddied with him, but was responsible for everyone. He can only herd so many cats at once, and chose to herd the two he could, and not the one that literally swam off.
You described him as a solo-diver in the OP, but now aren't which means a percentage of the responses are only applicable to what you initially wrote. It's hard to give advice or commentary when the story changes. Was he intentionally diving solo?
I have been diving for long enough (closing in on 40 years) that I'm pretty adapt at judging someone's diving abilities pretty quickly, both on the boat and, more importantly, in the water. As I have also previously posted, he wasn't a new diver but he wasn't very experienced either. I also noted that he'd never shot a DSMB prior to the dive in question. And, when he did try and shoot it, he lost it at depth.
He wasn't wearing a wrist computer so the only way for him to check his gas was by looking at a gauge which is pretty obvious and not at a glance.