Honesty requires flagging the evidence that cuts the other way. In the 1962 symposium Shaw the Villager and Human Being, assembled by Allan Chappelow, an orthopaedic surgeon named L. W. Plewes — who treated Shaw in 1950, when Shaw was 94 — recalled Shaw himself relating the exchange, and placing it before the first staging of Pygmalion. That is a secondhand recollection of a very old man, published after his death, and it sits directly against Shaw’s own signed denial of a decade earlier; but it is the reason Quote Investigator leaves the case formally open ↗ rather than closing it.
The reference works have moved. Richard Langworth’s Churchill by Himself (2008) included the anecdote, dating it to 1922 and naming the play as Saint Joan. Writing for the Churchill Project in 2020, after Churchill Archives Centre director Allen Packwood surfaced the Tatham correspondence, Langworth called the exchange fictitious ↗ and identified the play at issue in that 1949 correspondence as Buoyant Billions, Shaw’s last full-length play.
Note also that the popular wording is younger than the anecdote. The phrasing that circulates today — “bring a friend—if you have one” — is absent from the earliest tellings, which word the punchline differently each time: “if you really have a friend” in 1946, “if you have a friend” in 1947, “if you can find one” in January 1948. It reaches print in October 1948, in an address by Robert L. Stearns, and settles into its familiar form in Kay Halle’s Irrepressible Churchill (1966), p.116 — the version Quote Investigator took for its headline and the one Wikiquote reproduces.