air travel – Return cancelled after outbound has already been taken: what are my rights?

I’m trying to help out a stranded traveler in this thread. British Airways (one-leg) flight cancelled due to Covid

They flew to Delhi with BA but the return was cancelled due to Covid. I’m trying to find out what specific rules apply here. I wasn’t able to find anything that covers the specific case where the cancellation happens AFTER the itinerary has already been started. Everything I found on “cancellation” seems to assume that the cancellation happens BEFORE the outbound flight which either results in a rebooking or a full refund.

I would assume that the airline is required to bring you back, but that can be expensive for them if they have to rebook you on a different (non code-share) airline, so they will drag their feet. For a partially flown itinerary, refund is very difficult since it requires pricing out the individual legs.

Specific questions:

  1. What are the passenger rights ? If location is relevant, lets assume governed under EU law. What specific laws apply?
  2. Is there a time limit on how much later (then the original return) the airline can rebook you, regardless of how expensive for them it is? In this example it seems to be 4 or 5 days later.
  3. Can the customer instead ask for a partial refund and how would that have to be calculated?

Any other tips & tricks would be appreciated.