Hi Sunflower68
What I mean is that the dental practice would not have charged your OH privately 'by mistake'. There are specific clincial records card, treatment plan forms, consent forms, computer admin software for NHS/private patients.
It sounds as though your OH was not accepted as an NHS patient, and so was treated privately, given a private treatment plan and estimate, and thus charged accordingly.
The options that your OH has are to either go with the private treatment plan if he is happy to, or to look for NHS treatment at the same practice or another pratice.
PS: hope his infection clears up soon :-)