If she were just a diver I would agree but she is an instructor and as such should be leading by example even though she wasn't leading the dive, all professional divers are accountable at all times, the problem isn't so much her getting into trouble but a less qualified diver that might not be able handle an incident that she could of prevented had she been closer to them, fortunately this doesn't seem to have happened. As for solo diving that is a padi no no anyway.
PADI has endorsed solo diving through the "self reliant diver" certification. I am also a certified "solo diver" through another organization (although the card itself means little to me)
Also, I agree with your first point, but the next two points aren't exactly accurate. There have been several discussions about an instructor's "duty of care" on this website and others, encompassing what exactly an instructor may be held responsible for during a scheduled class, an unofficial mentoring session, or just going out for fun. I encourage you to seek this information out to avoid any misconceptions.
To your first point, though, leading by example is absolutely a responsibility of a good instructor. I feel that a good instructor accomplishes this by diversifying, taking risks, and continuously developing in their own way - but doing so with due diligence and understanding that with risk comes consequence. By clearly explaining the risk taking behavior, the motivations for doing so, the preparations involved, and the consequences of failure - you bring an open minded perspective that encourages learning and maybe even a bit of inspiration. I believe this sets a much better example than the typical card jockey instructor who teaches by regurgitating a flowchart of ineffectual certifications.
This is beside the point though. What is important to note is that a breakdown of buddy communication and dive planning is a failure at the
open water level. The responsibility is thus shared by all certified divers involved, and the fact that one of them is an instructor does not change that nor is it relevant in an objective analysis.
