Flights cancelled, but both the airline and OTA say I can't get a refund

MagicByNature

Back in May 2020, I booked some flights for my parents with Turkish Airlines, via an online travel agency. At the end of the year those got cancelled, and TK had a very generous policy of offering the ticket to be changed to an open ticket, which can then be used for a future flight with no extra fees, until end of December 2021. This later got extended to March 2022, then July 2022 and finally August 2022 (https://www.turkishairlines.com/en-gb/announcements/coronavirus-outbreak/flexible-change/). Throughout this time, I talked to the airline, making sure those tickets can still be used. 

Eventually, my parents decided to just get a refund, so I called up the TA who advised me that there will be no refund, as the airline informed them that the tickets expired in December 2021, and there's nothing they can do. I called Turkish Airlines, and they said they can't do anything (because it's the TA that manages the ticket), but also suggested that the tickets indeed expired. 

Of course chargeback/S75 would be the next step, as I booked it using my credit card - but as it turns out, Section 75 doesn't apply to flight booked for others (or through an agency), and chargeback can only be done within 120 days of the initial transaction. 

What am I actually supposed to do get the money back?