Problem with recently purchased car, is the garage acting correctly?

My parents bought a 1 year old car on the 2nd of June from a garage, having asked the salesmen whether it was suitable to tow their caravan, he said yes, it would tow anything so they bought it. 6 weeks later, the log book comes back from the DVLA, and it turns out the car is too light to tow their caravan. Parents phone the garage, they admit liability but because it is past 30 days they will only buy the car back which means they will lose 2.5k on a car which was sold unsuitable and the garage know they have erred. They have done 1500 miles in the car since buying it but even so does that equate to a 2.5k depreciation in 6 weeks!!!

Purchase price was 12k

Any advice gratefully received