Daughter-in-law was cheated to repair unreliable products. How to persuade them not to listen? What should I do?

Be calm, don't be impulsive, don't blame her blindly, and don't scold her rudely. Change the way you speak. Don't talk too much if you are not sure to convince the other party. The more you talk, the more disgusted she is. To "move with emotion, understand with reason", move with emotion, pull with emotion, pay more attention to her, express your worries, and remind her not to just bury herself in pulling the car without looking up at the road.