Absolutely not!
Bandwidth is not the only factor to consider in this. If he has the XBOX 360 hooked up over the wireless then one must assume that he is going to use XBOX Live with Audio chat. This means that there will be VoIP like traffic coming from the XBOX over wireless.
This is fine when there is only traffic from the XBOX but it all falls apart when there is traffic from the iMac. This is becuase two wireless nodes on a network do not communicate directly between each other and hence there is no way to know if another person is transmitting at any one time, especially since both of the nodes could be in range of the AP but out of reach of each other. Therefore WiFi uses CSMA/CC (Carrier sense multiple access with collision detection). When the XBOX tries to transmit a VoIP packet at the same time as the iMac wants to get a web page evrything on the network has to stop transmitting for some miliseconds and then try again.
This collision drasticly affects lag in the game and can fracture and ruin VoIP performance.
On a switched wired network however each node on the network can transmit and recieve in full duplex without any limitation, the scheduling of the packets is handled by the switch.
If you can wire it, WIRE IT
Chris