1.hardrest router ....reinput new user name and password...
2. set Qos in router ....remark you "friend" mac address ..it can set limited to the computer bandwidth (traffic)....!
or
inside router assign your "friend " computer mac as fixed IP (eg.192.168.11.5)....
then block ALL it's IP ports except port 21 and 80.......that he can only surf web and ftp...