Do you have MAC address filtering enable? You will probably need to add his MAC address to the list of "Allows". Also, make sure his wireless card supports the level of encryption you're using (WEP 64/128-bit, WPAv1 TKIP, WPAv2 AES).
Check that his computer has the IP address from the router, you can find this out numerous ways, but opening up command prompt and typing
ipconfig /all is the easiest. It can also help determine his MAC address (Physical address).