I wouldn't go the legal route to fix the flight - I doubt it ever works. In the immediate situation it does nothing except aggravate the airline representative.

In this particular case, my friend booked the ticket through expedia, DL told her to basically bugger off, and (probably because of the wide-spread European disruption) the expedia call center was completely unreachable (on hold for hours ... even their website was partially down).
AFAIK she is still in the UK, and will be spending Christmas there.

I would invoke the "European legislation" route, not now in the airport, but writing a mail from the comfort of my home after the whole thing is over. Not to solve the individual problem (hopeless), but hoping that if enough complaints are made, the airline will change its procedure in the future.
This is not a question of suing the airline - it is a report to an industry regulator about unlawful practice ... after putting a few pieces of paper in an envelope, I would never have to touch the issue again.

(And the regulation route does eventually work - there are some budget and foreign airlines in Europe that tried to systematically refuse compensation/rebooking/accommodation in the past citing their own "conditions of carriage", and after relatively few proven incidents, and the usual year or so of legal wrangle between the regulator and the airline, had to change their SOP or face a flight ban.)

Anyway ...
no use getting worked up - I was just surprised by the whole thing. Storing away the information.
My personal reaction to a DL guy telling me to contact expedia/orbitz because DL is not responsible would have been an outward big smile and a "surely you are mistaken", and an inward "and again they are lying to me and try to weasel out".