Is the agent liable if the principal discovers defects in the car the agent purchased for him, knowing that the agent admitted his error due to his lack of expertise, and is he obliged to repair those problems or sell the car and refund all expenses?

The wording of the question indicates that the friend was not an agent but merely pointed out and advised purchasing it. Therefore, he is not liable. A consultant is entrusted and incurs no penalty if he errs. The liability rests with the seller if the car has a significant defect, and the buyer has the option to return the item or accept compensation for the defect. The cost of transport and registration in case of returning the sold item is an area where scholarly opinions differ, but it appears that the seller is obligated to bear these costs if he was aware of the defect and concealed it.