Ask them who was the "faithful and discreet slave" back then. They might say that the GB acts talk about.
Tell them that there was no faithful and discreet slave in the first century. That whatever they did back then had no relevance cause Jesus didn't appointed them. Jesus had to wait till 1919 to appoint Rutherford and colleagues as the faithful and discreet slave (which they didn't know anything about, neither they didn't know that Jesus came in 1914 and they died without knowing it). And the GB was established till 1971.
They had to cherry pick a scripture to back up what they say. But look carefully in their publications, you will find "The Truth About The Truth" there.