I can only speak from PADI standards; I do not know what agency was being taught here, or if any others are less restrictive than PADI. But here are some notes on a few specific quotes, as I try to visual the details of what was going on:
The instructor asked me to help him and the other instructor keep an eye on the students.
That may sound reasonable, but it is very clear that PADI has exactly
zero standards for "help" provided by non-pro divers. A DM assisting the class is filling the role of a Certified Safety Assistant, and a DM is the lowest part of the spectrum that can function in that capacity. Beyond helping schlep tanks and other gear around, I can't think anything a non-pro diver can actually be allowed to help with.
One of the instructors and I headed out first to wait for the students to swim to us.
No indication here that a DM or Instructor was accompanying the students on the surface swim to and from the dive site. Clear violation of standards, if that was the case.
I called one of the instructors over when she said, "I can't do this". It took him less than a minute to get to us but by that time I was already struggling with her. .
This suggests that the Instructor(s) were delegating oversight rather than watching themselves, which is 100% the personal responsibility of the pros.
The other instructor and I got her calmed down enough to get her fin back on and I swam her back towards shore.
Sounds like another description of a student on a surface swim without required supervision.
Did this instructor pair you together with that girl? Then that is a different situation.
No, she was assigned another student as a buddy but he ditched her on the surface swim to us.
Another indication suggesting that the staff were not accompanying or supervising the surface swim, whether or not they were relying on the volunteer OW diver to fill that role.
Under the rules of scuba training, if an instructor is doing a training dive, then the instructor is responsible for everyone associated with the dive. Divers attending the dive who are not in the class count in the instructor/student ratio. If an instructor diving with the full limit of 8 students has a non-professional help keep an eye on things, then that instructor is violating standards by having 9 people in his care.
Exactly. If a non-student accompanies my OW class, they are subject to the same restrictions and responsibilities as a student, and I am now responsible for supervising the OW diver. (but they don't need to do the skills...)
If you and a buddy on your own dive want to swim around us and observe then you are not part of the group, and cannot be interacting with and/or distracting the student group.
So my key point is that those things that "seem to be reasonable" for an OW diver accompanying a student group become anything but reasonable if/when something goes south, and at least with PADI are explicitly not allowed.