"I Hope, Sir, that we are not mutually Un-friended by this Difference which hath happened betwixt us."

The earliest use of the seemingly recent verb "unfriend" was found by the Oxford English Dictionary way back in 1659 (in T. Fuller Let. P. Heylyn in Appeal Injured Innoc. iii).

Noted in "Language by the Book, but the Book Is Evolving/O.E.D.’s New Chief Editor Speaks of Its Future."