Maybe it must be considered from the perspective of the speaker. He may have a particular dentist in mind when he says "the dentist" and maybe the listener not having privy to it may not be relevant.

And he may be saying "a" about the word "nice" instead of about the word "evening".

Note: This is just my opinion and an attempt at trying to make sense of it. Purely subjective take as I usually refrain from making purely opinion based arguments.