Suppose you are talking to your friend about a third person called Mrs. Firstname Lastname. Suppose you are not on a first name basis with this third person; that is, if you spoke to this third person directly you will only address that person by their last name.
You to the Third Person: "Good morning Mrs. Lastname. How are you?"
But when talking to your friend (in the absence of the third person):
You to your friend: "I like Firstname. She is good at what she does."
Is this wrong? Is your friend correct to retort with the following?: "Firstname? Are you on a first name basis with her now?"
Merriam Webster defines first name basis as "having a close personal relationship where each person addresses the other by their first name". The key appears to be the phrase "each person addresses the other" which I believe you are not doing with the third person in the above example.
There is a caveat that I may be understanding the verb "to address" wrong, which the Merriam Webster does not appear to clarify whether it only means speaking directly with the third person, or it also includes speaking about the third person with your friend.
So is it still a wrong or unacceptable use of language to refer to a third person by their first name if you're not on a first name basis with the said third person?