How can one advise a relative to withdraw from an engagement to a girl, whose fornication has become known - while concealing it - without revealing her secret or arousing suspicion?

One of the most dangerous matters is to delve into people's honor, especially concerning immorality, because it leads to falling into prohibited slander (qadhf). If a girl commits adultery and repents, there is no religious impediment to marrying her. However, if she is still committing adultery, you are not obligated to warn a suitor about her unless he consults you. In that case, you may inform and warn him in general terms without mentioning the specific immoral act.