I've been flying quite a lot, but I haven't heard of this happening before - so, asking for advice here:

A friend of mine is trying to fly from London to New York with Delta.
She got her ticket through Expedia.

Her flight was cancelled, and Delta won't put her on another flight because "her ticket was booked through a third agent", and they basically told her to make Expedia deal with it, it's not Delta's problem.
She is still waiting for a flight out arranged through expedia, while Delta has half a dozen flights a day on the route.

I am very tempted to call BS on the reply of the Delta agent.
I know for a fact that this is against European legislation on Delta's part, if someone has a valid ticket for a flight here, the operator has a duty of care (including rebooking on first available flight, accommodation and food in the meantime, etc.) irrespective of where the passenger got the ticket from.
I rarely fly with US-registered airlines, and I don't know if this would also be against FAA rules ...

So, what is the forum verdict?
Can this happen because the ticket was booked through expedia, or did the Delta agent just Bull his way out and my friend was gullible (rather, is gullible - still stuck in London)?