A reader objects to the expression “a mutual friend”:
I don’t care if Dickens did write a novel called Our Mutual Friend. Using “mutual” to describe a friend you didn’t know was also a friend of someone else is a misuse of the word.
Mutual and reciprocal both mean, “directed toward each other.” For example, a “mutual admiration society” is a group of people who admire one another. “Mutual enemies” are people who hate one another. By the same token, “mutual friends” are people who reciprocate friendly feelings toward one another.
Many speakers—perhaps most—use the phrase “mutual friend” in the following context:
Sam is Joe’s friend. Sam is also Gloria’s friend.
Joe and Gloria are friends, but they don’t know that Sam is a friend to both.
One day, in speaking with Sam, Gloria learns that he just got back from a fishing trip with Joe.
The next time that Gloria speaks to Joe, she exclaims, “Guess what! We have a mutual friend.”
Purists would label this use as incorrect, arguing that the friendship between Joe and Gloria is mutual, but that Sam is “a friend in common.”
One definition of common is “belonging equally to more than one.” We speak of “common sense,” “common beliefs,” “common interests,” and “common complaints.” Another meaning of common, perhaps more prevalent in British usage than American, is vulgar.
Dickens could have named his book Our Common Friend, but he probably didn’t want the title to be interpreted to mean Our Vulgar Friend. The use of mutual might not have been strictly correct in the context, but its use eliminates misunderstanding.
The Chicago Manual of Style includes a caveat against the use of mutual to describe a “third-party” friend:
What is common is shared by two or more people: “borne by different mothers but having a common father.” What is mutual is reciprocal or directly exchanged by and toward each other: “mutual obligations.” Strictly, friend in common is better than mutual friend in reference to a third person who is a friend of two others.
The OED has this to say about the use of “mutual friend” to mean “friend in common”:
This use has in the past been censured as incorrect but it is nevertheless frequent. It has probably been used in preference to common on account of the ambiguity of the latter (which in many contexts could also mean ‘ordinary’, ‘mean’, or ‘vulgar’).
In my view, objecting to the use of “mutual friend” in the sense of “a friend in common” is officious nitpicking.