There is a plethora of information in all these Posts abt BA's 'obligations' when they cancel flights

I can't find a clear guideline abt what happens if BA cancels the Return-to-Home flight on a 2for1 Booking--is BA obliged to book with any--eg Air France-other Airline
or are they just obligated to return the unused Avios----and leave stranded passengers to their own device (and expense) of buying expensive one-way tickets home with another carrier who is flying the same route??