Basically it is hard verging on impossible. As you don't have access to the user database there is no way to import the users, you would have to get every body to sing up again.

The posts and threads. It would be "possible" to write a script to import these all but not very easy i would think. In the end copying and pasting everything in is probably going to be the easiest way although not the most pretty and quite time consuming.

Unfortunatly can't help you any more than that but warn your friend that the longer they leave it the harder it gets!