Clan Campbell Society of North America website does list it as a sept, however says...

"I can discern no reason for the inclusion of this name among the septs of Clan Campbell. Black who derives the name from Mac Shealbhaigh - ‘Son of Selbach’ - gives various instances of the appearance of the name but they would seem to centre on Galloway if anywhere. The local telephone Directory gives five instances of the name in various parts of Argyll.

Under Macelvie or Mackelwee, all forms of the same name, Black gives mention of the existence of the name M’Ilwee in Bute in 1656. The name also exists as McElwee"

http://www.ccsna.org/septs.htm#A17

So I guess some family history would be in order for your friend in order to determine where is roots originate from.