Travel start refund

We booked for our flights through travel start in January 2024 for our holiday on the 24th March 2024, our booking reference was ZA***9 or JN076V On British Airways..my husband and my son as well as my self hold EU passports but my sons girlfriend Ms.Tasneem Maasdorp has a SA passport, prior to booking on British Airways I went on to their visa requirements because I knew that she may need a transit visa, I looked aswell on the gov.uk website just to make sure. On both websites it said that if the traveler was visiting a schengen country and had a flight to the schengen country and was in transit for less than 24hrs and wasn't leaving the airport or going through passport control to get a connection elsewhere in the UK a transit visa was not necessary if the traveler has a valid visitors tourist schengen visa, so I booked our flight on British Airways , when we arrived at O.R Thambo to get our flight, Ms Tasneem Maasdorp was refused to board the flight, the ground staff that were checking the passport didn't even check our flight details or our connecting flight they just plainly refuse to listen to us and denied her boarding the flight stating that she needed a transit visa, which I double checked that she didn't. They canceled her flight and I had to go and buy another air ticket through another airline for her to go on holiday with us. I sent a request to travel start and they refunded me R5900 of the R14300 I paid for the flight, I feel that I should get my full refund back as it wasn't my fault that her flight was canceled, I had all the proper documents required and they didn't assist me in anyway. Someone somewhere is missinformed. I emailed British Airways and they say that since I booked through and agency and not directly through them that the agency should sort out my problem. It's been a back and forth the whole time and I really hope someone will sort this out

I ask that you please look into this matter for me and refund me the balance of what I paid. Claudia