I have only ever gone into another owner's stall for something other than an emergency and that was to put the horse's blanket back on when the owner refused to do it as "that was the barn staff's responsibility." Given that temps were getting down below 0 that night and the horse had a very thin coat, I made the decision to put the blanket back on (I did report that I did so to the barn manager however).
It takes a lot of brass buttons for anyone to go into a horse's stall and start working a massage or even more, what is the word, chutzpah, for someone to call in a chiropractor. On top of that, I wold be curious to know what story she told the chiropractor in order to get them to work on a horse she didn't own...that could be cause for them losing their license to do work on a horse that wasn't the property, or responsibility, of the person requesting the service..the other question..was the chiropractor actually licensed?
No, I would say you are not over reacting and agree with others that I wouldn't have been so, tolerant (for lack of a better term) of this person. She would have known even before I went to the manager how I felt about her interference.
The barn owner/manager needs to seriously consider asking this woman to leave...has there been any evidence she has been doing this with other horses? You could have a harassment claim as well to use as more leverage to get her to leave you and your horses alone.
Natural horsemanship has its place but with anything people can become so fanatical they lose all touch with common sense..at that point the practice (any practice where people can become fanatics) becomes dangerous.