What should I say to the bride's family regarding my friend who wishes to marry, knowing that he has a bad character, but has repented and regretted a previous sexual relationship he had with a girl without committing zina? And is there sitr (concealment/covering) in such a case?

If the young man still has bad manners, the suitor can be informed that he is not characterized by good manners. However, regarding the forbidden relationship that the young man regretted and repented from, it is not permissible to mention it, and he must be concealed; because the one who repents from sin is like one who has no sin, and concealing a Muslim is required.