I think the question about whether her "trained" to do that is very valid.. in fact, it needs to be answered before any speculation on why he did is valid.
We know that one handed actions are more intuitive when we operate at extension.
As for what we should do, if you CAN use two hands to control the gun, both alignment, and more importantly: recoil, I think you should. If you can't, then, by definition, it will be because there is something else going on with the other hand... which means that there is little value in planning on what to do with it if you are not using it to help the shooting. The circumstances will dictate.
-RJP