he claims " it is the only organization on earth that preaches the good news about God's Kingdom."
This simply isn't true. First of all, there is nothing in the bible stating that any "organization" has to be in place in order for people to preach about God's Kingdom. Second of all, there are certainly other Christian denominations that preach about God's Kingdom or Christianity in general. Just because they don't go out knocking on doors Saturday mornings, doesn't mean that others aren't preaching. The other Christian religions have simply taken advantage of modern technology and use the TV, radio and the internet to preach, which makes sense. You can have 20 million people watching you preach on TV as opposed to the couple of dozen doors you might knock on Saturday morning, with no one home. So who's doing the greater work here?
Second of all, I would point out that the notion that first century Christians went "door to door", never happened. The scripture they use in Acts is referring to the Christians actually meeting in their respective houses (like a bookstudy), each week. There were very, very few wealthy Christians in the First Century, and they did not have a normal 'meeting place', so they met in their various homes. There is not one, single reference in the bible to anyone, including Jesus, in the first century going "door to door" preaching. They always went to public places and spoke, or did 'informal witnessing.' And they certainly never had to turn in their 'time' to the congregation.
Lastly, the bible clearly says that not all members of the church or congregation would be preaching:
1 Corinthians 12:29 Not all are apostles, are they? Not all are prophets, are they? Not all are teachers, are they?
Many other churches, including the Catholics, Baptists, Mormons, Protestants, Lutherans, Methodists, etc. all have Missionaries. Witnesses certainly do not have the exclusive on that. And guess what, they all preach about Jesus and God's Kingdom.
Here's a nice little list you could give this guy, to show some more well known people who have converted to Christianity, without the help of the Witnesses, which means someone must have been preaching to them:
http://en.wikipedia.org/wiki/List_of_converts_to_Christianity