Shocking sale of dud and won't take ot back.

My son bought FORD RANGER 2.2TDCi XLT A/T P/U D/About 18 months ago. It has had endless problems from day 1. They sold him a dud and then had the nerve to use HIS warranty to fix problems they should have sorted out. They continue to delay on taking the car back and are now saying he will owe R50k. Constant delays in booking it in and fixing it for the umpteenth time. Wendy Knowler can you assist? This is blatant abuse of consumer rights. Unacceptable! Do not buy from them. They have not displayed integrity with a young man, with not much resources.