You've had a lot of reinforcement of your decision to advise on getting focused help. In case you are asking a different question, here's another take on it. I have DMed divers with experience but using new gear who have looked similar to what you describe. In those instances and I'm guessing in the one you describe, the issue is weight distribution and the diver's ability to recognize problems with it.
I have been able to fix this relatively easily by moving the tank up or down in the straps, adjusting the harness if it's a backplate, using a weighted STA or a heavier plate (again, if it's a backplate,) and moving weight around into pockets or onto tank straps - mostly, getting weight off of a weight belt, which tends to drive the center of gravity toward the feet. Vest-type BCs are harder since once the air moves up to the shoulders, it takes a little effort to re-distribute it. If he got enough air into the BC to be neutral but was hanging vertical, this sounds to me like the same situation.
Since he is a newer diver, he likley just doesn't know what he is supposed to be feeling in the water and won't until he gets some more dives in.
So if your question is "what should I try if I see this again," those are some of the things I would try. From DIR-F, you know about arching your back and tightening your butt to get the feet up, and you can tell a diver to do those things, but ultimately they won't overcome grossly mis-distributed weight.