Do you have any other information to help us help you? Games he already has? Games he likes? Does he have a new powerhouse PC, or an older one?
L4D effectively doesn't have a single player mode. It's only really worth playing with friends cooperatively or in versus mode. With no information, it's difficult to offer a reasonable recommendation. Specifically what games he has already played. It would be hard to go wrong with the Orange Box, however. It normally goes for $30, but I bet you could find it for less.
http://www.amazon.com/Orange-Box-Pc/dp/B000PS2XES
Some other titles you can look at:
Painkiller
Return to Castle Wolfenstein
Quake 4
Doom 3
Original Half Life
Far Cry