Show him that and I fully expect he will come back with Luke 11:23:
He that is not on my side is against me, and he that does not gather with me scatters.
He will not see the profound irony in this. Apparently Jesus has a very poor memory.
In seriousness, a possible explanation for the contradiction, within Christian theology, is that in chapter 9, the man was working in Jesus' name, just not associated with the band of disciples. While in chapter 11, he was referring to being on HIS side, i.e., believing on him. Of course, this explanation would make JWs uncomfortable, since it could be taken to mean that Jesus is unconcerned with what brand of Christian you are, so long as you believe on him. Seems a nice logical explanation to me, but hey, what does a Pagan know about Christian theology.