What should I do when asked by the family of a girl whom a friend of mine, whom I had previously praised to them, proposed to, but I later discovered his bad character, knowing that the people of his hometown only know good things about him?

If you are asked to advise someone who seeks your counsel, it is incumbent upon you to inform the family of the girl about what you know of the suitor, whether good or bad, if you have certain knowledge of his current state. However, if you do not know his current state, it is better to excuse yourself from disclosing his condition due to your lack of knowledge about what he is currently like. You can say that he previously had some shortcomings, and that you do not know his current state and perhaps he has changed. It is advisable for them to inquire about him from someone who has recently been in his company.