They wouldn't have been turned away at check-in though if they OLCI'd or used SSCI and no baggage to check-in.
That's why I said "or at the gate".
And do the GA's really check passports properly anyway? - they only look at the photo-ID page and not whether the passport has the correct stamps or visa entries IME.
They do with non-EU passports. I've seen someone held at a gate once while further enquiries were made.