Is it permissible to joke with girls by their saying, "I got engaged," or "I will return your dowry," and is it considered forbidden not to comment on this joke, which contains a lie, and to continue the joke, especially if it is public on Facebook, where it might be difficult to offer advice?

What the questioner mentioned is forbidden for two reasons: The first is that it contains lying, based on the Prophet's (peace be upon him) saying: "Woe to him who narrates something and lies to make people laugh thereby; woe to him, woe to him." The second is imitating men. The imitation of men by women is forbidden, based on the Prophet's (peace be upon him) saying: "The Messenger of Allah (peace be upon him) cursed men who imitate women and women who imitate men." This imitation includes appearance, clothing, speech, and gait. If the prohibition of this conversation is established, then it is obligatory for anyone who sees it to denounce it and forbid it, and for anyone involved in it to repent. If the reprehensible act occurs publicly on Facebook, it should be denounced publicly on Facebook, based on his (peace be upon him) saying: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart — and that is the weakest of faith." The denouncement should be done gently and gradually, after clarifying the Sharia ruling.