I want to chat with her about a common friend.

They would often talk together about the music of their common friend, the musician Elgar.

Chembai met him in 1924 at the house of a common friend, and became good friends.

They were introduced in a restaurant/bar by common friends.

I had never heard her more malicious about our common friends.

For example to express that John and Mary have a common friend, one can write.

About a month later, I met him at dinner at a common friend's.

A week before departure I learned through a common friend that a single man was taking the same tour.

It was impossible for her to have forgotten to feel that this arrival of their common friends must be soon bringing them together again.

In the 1988 Presidential campaign version, the answer might well be "common friends."