Water cap

Good day Please advice 2 week back on Saturday I was attending the funeral on my way I passed by garage for petrol then petrol attend check my water and oil, while I was driving about +- 30 kilometers the car went dead on us in the middle of the road...... We have to call insurance to taw the car to mechanic, on Monday he told us the car was opened the water tank while the engine was hot that is the cause and we will have to buy new engine which everything is cost me R45, 000.00 then we went back to the garage to explain the matter to the owner he doesn't want to listen to us he said yes he checked the footage petrol attendant he only poured water it is not his problem about what happened, but argument is he was not supposed to open the cap please give me some tips..... Below is their physical address Sasol garage Cnr. Rossini Boulevard &, Barrage Rd, Vanderbijlpark S. W. 2, Vanderbijlpark