The letters do generally feel written right and the language used is proper and something you'd get here. What is a bit strange are the typos in the mail from the insurance company, but who knows, the people writing those aren't always the brightest bulbs.
I looked the facility up just now and the name of the doctor from the letter lines up, it is actually only an ambulant facility though, not a hospital, no stationary beds. According to their website, they send people that need that kind of care to a nearby hospital. If she had a stationary stay, then there would need to be a bill from some other place on top.