I haven't been in this situation yet, but these are the things I would say. Of course in a humble, gentle, non-confronting tone:
"The GB says in the February WT that they are not inspired. They also said that they are not prophets, and that the GB consists of imperfect men. Yes, I agree with all they said about themselves!
(But I'm interested in how exactly the GB is led by Jehovah. They do pray for the Holy Spirit, don't they? And can't I pray for the Holy Spirit too? So where is the difference?
And if an other religion changes its teachings or makes mistakes, we point out that therefore they cannot have been led by God. But if a householder brings up that we have changed our teachings, we say that it is New Light. Can't those other religions say the same about themselves? How can we show the householder the difference?)"
Another idea:
"What do we need to understand the bible?" - "The right attitude and the Holy Spirit. Oh, and the Faithful and Discreet Slave." - "In what organization was Russel? Who helped him understand the bible?"