My family flew in
Kiev-Paris-Dublin to visit us in Ireland, end-to-end return flights booked with AF-KLM.
Unfortunately, they were stopped at the boarding for the Paris to Dublin flight, due to an Irish visa issue that wasn't spotted by anyone until that point of time. The AF agent in the airport suggested they speak to an Irish embassy and try to rectify the visa.
The visa was fixed and they are in Dublin now, but the KLM customer service says that they will need to pay a large (circa 250 EUR) penalty fee to be allowed to fly back on their return tickets, since they didn't use their journey to Dublin, and that they can only complete a (possible!) check-in for flight in the airport itself.
Is that sort of penalty legal and what can we potentially do to avoid the fee/make sure they can fly tomorrow?
EDIT: Went to the airport, AF/KLM told us to pay 250 EUR per person to fly the original return tickets. Time was unfortunately very tight, so we ended up going with this option.
Also, if we had any idea this was the case even a day or two ago, we would have just flown with Air Moldova flight as suggested in the answers.
Overall, they followed all recommendations of the airline staff, starting from their suggestion to remain in Paris to fix the visa, and still ended up paying a huge fine for no reason at all.