Having a hacker over, risks?

[castlevania]

A hacker friend has invited himself into my summer house. He asks if he can stay for a week. Since I am a bit paranoid and he isn't very clear cut morally I'm a bit worried that he will hack in to my computer. I have valuable information on my computer that I don't want him to take part of. So I'm asking you guys what risks might be associated with having him over for a week. He is very smart and knows amazingly lot about computers.

The network setup would look like this:

...............Internet........................... ........
...................|.............................. ........
........______router/modem________........................
...........|...................|.................. ........
...his computer...........another router (Linksys WRT54GS)
...............................|.................. ........
..........................my computer.....................

Would he be able to hack in to my computer and get hold of valuable information?