Turns out that, when friend tried to check in online - the App said No. Thinking it was just BA's usual rubbish IT, logged into the web account. The return booking was absent. When entering the locator code, "All segments of that trip have been flown" (approx words). As this was some six hours before departure, it seems that BA knew well in advance that the out and return rotations would not operate. Reason given at the airport was 'Bad weather over Heathrow' which is valid to some degree but when?