It wouldn't necessarily cause a problem, as long as its on a channel that isnt interfering with the Arubas. One thing to bear in mind is that it will get classified as a Rogue so if you have WIPS policies enabled, you may end up blocking it from working.
Sensible question to ask is why can't he connect to the Arubas? Can you offer him his own SSID in that area? He could be doing all sorts on your network that you may not want!