Never tried it but ....
Set up a 2nd WiFi SSID and VLAN under Wireless Settings/ Channel and SSID and give it a diff password under WPA. You will need to change your original pwd of SSID1 so he can't get to it. Give him the pwd for SSID2.
Under Bandwidth Control, share out your total bandwidth between your SSID1, majority, and his SSID2, minority.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC