You did not indicate how full F ended up but assume light load from your comment about pax in gate area. It may well have come down to loaded meals in that case, and yes, seems like you were shafted. A full F cabin with 10 pax will have a couple extra meals laoded. If there were only 3 pax they often only load 3 or 4 meals, so her 2 buddies would tip the system out in that case.

I frankly would have politely asked for the torn up BP "as a souvenir" looking the agent right in the eye. On what basis could she say "no"? Yes, old seating will be on reloc archive, but sending THAT boarding pass to MP along with "new" pass is pretty graphic proof you were done over.

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~ Glen ~