The case you have presented in your inquiry needs to be looked at from an “enjoining good and forbidding evil” perspective.

Although enjoining good and forbidding evil (amr bil ma’ruf wa nahy anil munkar) is wajib for all Muslims, nevertheless it has different stages; the first of which is to speak with the sinner himself about the wrong act and remind him of its bad worldly and otherworldly outcomes in a decent manner and to ask him to put it aside. Inshallah this encounter will awaken him and prove effective. If this doesn’t work, you can threaten to inform his/her family of what is being done and hopefully it will work. All of these stages need to be done with gentleness and moderateness, not harshness. Of course, if there are chances that informing his/her parents will entail greater problems and grave outcomes, then it is no longer your responsibility to tell them anything.