in the case of your friend his itin would be cancelled in most likelihood. however it has been my exp with OW that if you are connecting between AA (the only choice between Canada / US now) and CX is that they are not aware of each others reservations. so if your friend is a gambling man -- if caught worst case is that they cancel his tix and charge him the new fare and since he wouldnt have used any of the segments he wouldnt really pay anything extra beyond the fare difference -- so if the diff is big enough he may want to give it a shot.

having said this i wouldnt reccomend it just b/c of the hassle and also the fact that seats may not be avail when he has to reissue the ticket in the US.