I think that is a pretty sad situation that people who are uncomfortable with drugging horses are being advised that they shouldn’t bother training at a hunter barn or aspire to show in the hunter divisions. It is a terrible marker of what has happened to hunter and equitation judging that so many people accept that sedatives are a matter of fact part of those disciplines.
Now, on to the OPs situation. If I were in your shoes, I would curb my emotional reaction to what happened and have a matter of fact/information gathering conversation with the head trainer at the barn. I would mention what you saw and discuss your concerns. If the trainer had a reasonable explanation–and I can’t imagine what that would be but who knows–then your mind can be set at ease. Otherwise, I think most likely you probably would be more comfortable looking for another trainer who is more on the same page with you.
I think there are plenty of situations where giving a horse a sedative for handling or riding in a private situation is acceptable. I don’t care if someone wants to give their (non-competing) flighty horse some acepromazine for a ride on a windy day after a three week vacation, and I certainly wouldn’t hesitate to sedate a layup for hand walking or turnout as advised by the vet for safety of the horse or handler.
However, I’d be uncomfortable training with a trainer who was comfortable giving a horse acepromazine or any other injectable sedative in the manner and situation that you describe. Obviously giving a horse a calming agent for a video that will be used for a competitive or sales purpose is unethical. Also, I think that using medication to hone performance over fences, is neither safe nor appropriate. I personally would not want to ride in a jumping lesson on a horse that had been given an intravenous calming agent. A parent who allows their minor child to do so obviously hasn’t thought things through.
Now, just FYI: in the hunter world some people (trainers) genuinely believe that they are engaging in clean sport if they use things that can’t or won’t be tested for, things like injectable magnesium, or drugs that are allowed for other reasons but also are thought to have a calming effect, such as dexamethasone, etc. So your trainer may genuinely believe that she runs a clean barn because she is careful to only do things that she wouldn’t get caught for.
It IS possible to do the hunters in a clean manner. You may need to look for a new trainer.