Originally Posted by
SilverChris
The airline flying Turkey-CPH may not view your friend as a transit passenger, because as far as they're concerned, their obligation ends in CPH.
This is the likely answer right here. Even if you have an onward BP on a separate ticket I'd say there is a reasonable chance that the airline running the first leg/ticket would not let you board anyway without a valid passport / visa for the first destination.
That and AC BP generation is ... somewhat less than reliable. I had one recently that was the opposite: the app gave me two BPs but when I checked in at the counter they could only give me the first one.