I am not too sure why you should think that BA should have any responsibility for your luggage when you did not travel with them.
Nut Phileas did travel with BA. That is who he paid for his ticket and that is who his contract was with. If BA wishes to sub-contract the flight to someone else then fine - but Phileas' contract remains with BA and it should be up to BA to recompense Phileas and then claim it back from their sub-contractors.