It really depends though. If he has no password but he is blocking all but specified MAC addresses then having a password is less important. However, if he just has his wifi wide open with no mac address restrictions, then he could be in for a world of hurt.
I'm not saying I did this but....
A few years ago my in-law's neighbor had an open wifi and someone connected to their network, logged into their router, added a password to their wifi network and changed the admin password on them thus locking out all their wireless devices from connecting.
Let's just say they had a new password on their wifi the next day.... I mean, errr, someone told me they added a new password the next day.....