Break this into two itins: one an open jaw with YYZ-LAS outbound and DFW-YYZ inbound. I doubt any single carrier does both these nonstop (well, AC might) so you'll probably be changing planes somewhere on both legs (and if you fly AA your outbound might coincidentally be YYZ-DFW-LAS, although more likely YYZ-ORD-LAS).

Then for the LAS-Dallas leg fly LAS-DAL on Southwest. Because of the Wright amendment you have to buy two separate tickets, most likely for LAS-ABQ and ABQ-DAL. Upside is that WN doesn't hose you on one-way tickets (they cost half a roundtrip). You may be able to get one or both of these legs for a base fare of $30, depending on which markets WN is offering $30 fares between this week. Your friend must be willing to fly WN, of course, and s/he has to line up twice for boarding passes.

Hey, you did ask.