Thanks to Dietrich Jung for bringing to my attention Ignaz Goldziher's recollections of Afghani, with whom he spent time during his four-month visit to Cairo as a young man in 1873/74. Goldziher described Afghani as "the most original figure among [his] friends" in Cairo. He joined Afghani frequently in the coffee house in Abdin Street where Afghani "presided every evening over a group of young Azhar students and taught them all sorts of free-thinking stuff (ihnen alles mögliche freisinnige Zeug vormachte)" (Tagebuch, Leiden: Brill 1978, p. 68).
Goldziher wrote that he met Afghani again in Paris in 1883, and should one day write more about this. but apart from an entry in the Encyclopedia of Islam, seems not to have done so.