If you think $1,000 is a lot of money for several chiro visits a week, you oughtta see what the bill would look like if she went to an orthopedic M.D. for the same treatments... would probably be closer to $7000 a week. And just the x-rays alone at a regular hospital or radiology group would probably be close to the $1,000 right off the get-go. Radiology services can be ungodly expensive (having worked for a major northern California radiology group for 2 years, I can tell you it's WAY expensive.... not just the x-rays, but then the subsequent bill from the radiologist who interprets those x-rays).
If she was already seeing the chiropractor, obviously she already had spinal alignment issues and heaven only knows what else going on. If a patient is already having issues and is then even in a minor accident, which can include even small twisting injuries in the home, it can (and usually does) exacerbate whatever else a patient may have going on in their musculoskeletal system. Unless you are an M.D. and/or have first-hand knowledge of any on-going medical conditions she had prior to hitting said deer, I would not be so quick to judge this chiropractor. I am guessing that the insurance company is trying to get out of paying the bill, which is why they are questioning it. I don't know if you know this or not, but healthcare costs are through the roof, and insurance companies do everything they can to get out of paying a patient's medical bills nowadays.
In regard to your comment about "new to us as she still works".... if she is injured and in pain, her visits to said chiro may be the ONLY reason she's even still able to work without eating handfuls of pain killers every day, which is what a traditional M.D. would prescribe. Be grateful she's getting this kind of treatment and not gorked out on Vicodin.
In any case, I send your sis my best wishes for a smooth and uneventful recovery. And tell her to keep seeing that chiro if the treatments are working for her keeping her functioning!
ETA: If anything, I would report the insurance company for discussing her medical condition with a 3rd party (the boyfriend), because that's a violation of federal HIPAA laws. They cannot even mention to a 3rd party that she SEES a physician of any kind, chiro or M.D. or otherwise, and certainly cannot legally disclose the name of any provider that is treating your sister and submitting bills to the insurance company for payment. This sort of thing is exactly why HIPAA was enacted, and that insurance company is in gross violation of that law. I'd report 'em to the feds for discussing it with the boyfriend at all if I was your sister.