Well, I cannot say that I disagree with Bingo or Plashba -- Expedia certainly has a huge responsibility to make good here.
However, I believe that when a ticket is purchased, it is purchased under the ticket stock of one of the airlines -- in this case LAN. LAN likely collected the money directly (Expedia merely acted as an agent -- and got a commission as well as fees from the buyer). I therefore think LAN has a huge fiduciary responsibility to make good on a ticket purchased under their stock and by their credit card processing.
Of course, LAN may not work this way and who knows who processed what charges, but if the ticket was on LAN stock and ticket numbers then I think they have the majority of the responsibility to make good.