When Catholics know of sins of another, we admonish them privately. If they do not heed the admonition, we bring another. If they still do not listen, we bring the Church (if we actually have access to Catholic clergy). If then still they do not heed the admonition, and their sins are of such a nature that they are likely to cause harm, injury or loss to another person, then of course it is not sinful to reveal the sins, as a spiritual act of mercy, that the person may be punished and corrected by the lawful authorities, and so no one else may be hurt.
This must all be done with a view to helping them.