How to transfer 1 of your sites in your network to an external hosting provider?

I am running WP Multisite. SO far so good, but I just thought of a possible problem in the near future that I wouldn't know how to fix.

All of my sites in my Multisite installations are for my clients.

My question is: If one of my clients decides to take his website (all its contents) to another hosting provider, how can I provide this to him??

- I don't have a folder for each site created in the Multisite (don't know if this how Multisite normally works of if I made a mistake on the installation.
- All of the site themes and plugins are installed in the network, not on the specific website.
- In other words, I don't have nothing to transfer except the posts, comments, pictures, etc. But the website design, the wordpress installation I can't do it.

Your light will be greatly appreciated on this issue. I'm sure you can point me towards the right direction.