Help requested on behalf of a friend. Was on a BA ticketed LHR-BOS return ticket a month+ ago. Return flight on the 19:10 A380 service, down-gauged to 777 due to the usual issues on the 380. Not accommodated in that change but moved over to the AA service departing at 20:20. Then the AA 777 goes tech and departure ends up being 02:00 as they had to fly a replacement aircraft in from JFK. Final arrival time at LHR is around 14:00 UK instead of originally scheduled (BA) 06:20 or so.

Passenger applied for compensation and was rejected by BA as "it was an AA problem". I suspected this would be the first reaction so seeking guidance on who they should pursue with? My thinking was BA ticket and original BA problem, so BA - but is that right and how should I advise them to proceed? Thanks!