From what I know of insurance talking with my dad, the HR person doesn't actually now where they charges are going to, just that there is a large charge. For example, last year the company he works for had to change insurances because of three large expenditures. The HR person know who these three expenditures came from, but did not know what they were for exactly. She didn't know that two of the charges were from my MRSA diagnosis and following hospital stays, only they they were connected to me.
As for being fired, he has ADA and FMLA protecting him, as long as his company employees 50 people in a 75 mile radius. Once he has been at the company for a year, he can have, I believe, something like 60 days off from work a year as long as he gets a note from his doctor explaining he was sick from disease related illness. The company might send him to their own doctor for confirmation, but he cannot be fired for that time off.
This is all information my father has told me after discussing our own situation with the HR person at his business.