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Assuming that you and your friend are both able to obtain J award seats on the itinerary, your friend can redeem BA Avios for the same award itinerary. However, your friend will pay BA's redemption rates (which will be higher than AA's), and will also have to pay fuel surcharges on all flights, except AA flights within the Americas, QF domestic flights, and LAN flights worldwide. Depending on the exact itinerary, your friend may have to pay significantly more in fees than you will, even though you'll be on the exact same flights.