What should I do about an immoral girl whose situation I have come to know? Should I tell her friends, who are unaware of her true nature, or should I remain silent for fear of causing her divorce, or should I inform her family?

You have done well by staying away from the company of that deviant girl, because a companion influences their associate. The advice is that you should first counsel her without revealing her secret. If that does not work, then inform her parents of her corrupt morals if you expect benefit from doing so. If her parents are of no avail, then report her to the authorities of enjoining good and forbidding evil. If you do that, you will have absolved yourself of responsibility.