Originally Posted by Martin Sparrow

Well you can simply remove his old site.
And place his new WP site on his original domain.

The built in 404 page in WP will catch all the old incoming traffic to pages that don't exist anymore.
If you want to go fancy make that 404 page with a nice text, ad a link to the homepage and maybe ad a search box (if it doesn't already has that).

Does the new domain already have traffic?
If so, just .htaccess redirect that to the new domain.