The condition with the thermoswitch in the compressor takes a long time from where it happens only occasionally to becoming common even at lower ambiant temperatures. I suspect the person your girlfriend bought the car from knew about the problem and it was probably one of the reasons for him to sell the car. Go to the local Volvo dealer or the dealer that sold the car new and ask them to print out a list of service they performed on the vehicle. If you find that the seller was advised of the problem and he did not tell your girlfriend when she purchased the car you have some remedy. I am not a lawyer but if I found out the seller was not truthful of a known defect I would first contact him and try to work out a deal for him to participate in the repair. If he refuses I would then contact an attorney. A new compressor cost me $880 dollars installed at my Volvo dealer for the thermoswitch problem. I would expect the seller to pay at least half because you will end up with a new compressor. That is a small price compared to legal fees (both yours and his) court costs and what ever damages the court awards you. It does not pay to hide a defect when selling a used car. The courts will hold the seller to the same standards as a professional car dealer when fraud is determined.