I wouldn't do it but it is possible to setup 2 networks. Without buying extra equipment I wouldn't feel safe sharing.
You could set him up on a DMZ with a different subnet mask as yours, lock down your internal network with a hardware firewall and not use wireless at all, and put a hardware firewall between his DMZ and the internet. This way it will be harder for him to reach your internal network (not impossible) and if someone was attacking your connection it would go to his box first. Just another layer of security.