As a recently certified diver, I would have been completely perplexed by the other instructor's actions, and quite likely completely pissed off at him.
If the other instructor felt that I was somehow crowding his student, he could have - and should have - made reasonable attempts to communicate that first and foremost. I would expect an instructor to carry a slate, and he could have written a quick note explaining they needed more room (if that was the case).
If the other instructor could not convey what he needed to the student, he should have made attempts to convey it to the student's instructor, and let that instructor get his student to move out of the way.
Pushing a student off a platform is irresponsible at best. It's a good bet I would have been in the quarry's dive shop, asking to speak with the manager or owner about the actions of his instructor. This instructor represented his LDS poorly, and as litigious as our culture has become, he could have opened himself and the shop to a lawsuit. Even if the suit was seen as frivolous, it presents an expense in time and resources to deal with.
He also had no idea what kind of person he pushed. I've known a number of people that would have waited topside for that instructor after the dive, and confronted him - possibly violently - for his actions. Such an overreaction isn't right, either, but it's a reality we read in the news frequently over such trivial things as parking spaces, the latest hot toy, etc.
If you have any kind of rapport with the other shop owner, my suggestion would be to call him and express your concerns about what happened, from a position of concern over how it can affect his business as well. If the offending instructor has some explanation, it might help clear things up (maybe he was feeling your student was crowding his, and his attempts to convey that were being ignored, and he felt he had to do something). Starting out from an objective position, seeking to cooperatively investigate an incident, is always a good idea. It's easier and more effective to become a hardnose later if needed, than it is to start out as a hardnose if it's not needed.