For IATA airlines it is always the late delivering carrier that is responsible for fixing things. E.g if it's an AC operated flight AC is always responsible for doing ng what is needed to get to your final destination -- it doesn't matter of it was booked as an AC or UA flight number. This is generally true of any interline itinerary, not just within the alliances, e.g. if I book an OM ticket connecting to UA and OM causes a misconnection OM would have to figure out how to get me to the destination.