YYZ-LAS is a vacatio/convention route, so fares are likely to be less than YYZ-DFW, which is primarily a business one. I'd buy one ticket [AC, UA or AA depending upon FF program your friend belongs to] with Saturday overnight, 7-day advance. Then book, as suggested, a cheap SouthWest to DAL. Make the return trip from DAL so it arrives a couple of hours prior to the AC [UA/AA] departure from LAS back to YYZ. Of course, if this has to be done next week, your friend is out of luck and should book this trip with points.