I agree with Theo's comments completely with the following additions.

In my first post on this topic I used the term "a trusted friend." I don't use that term lightly. One of the reasons new friendships develop over a period of time is because there is a bit of a "trust probation" period. Rarely you will find an individual who upon meeting you put the complete and utter trust in you. They'll usually start off by putting faith in you (kind of a "trust but check" mentality). With a good track record you eventually can move from a "faithful friend" to a "trusted friend."

Nothing kills a new relationship like interjecting your negative feelings about others. Until they trust you, they will question your motives and likely view you as hyper judgmental. How do you know when they start to trust you? Well, they start asking more direct questions. "What is you honest opinion about THEM?" Even then you still need to protect yourself with Theodora's more neutral comments. If they ask you second and third level questions, if you reluctantly and with caveats provide them insight they will more readily listen to and respect you. I didn't find out anything about the Caer Mear "Summer of Love" until I was 8 months into the SCA and then it was given to me in the above manner. I was very thankful for that approach.

Here is my one caveat. If the THEM in your case have or are actually violating society, state or national law in a manner you feel endangers them, I'd say you are honor bound as the people's protector to warn them. If the person is a minor, you need to tell their parents or guardian and let them make the decision.

Here are two extreme and unrelated to your situation examples:
1) Lord Bad Boy has been charged with child molestation and it's currently in the courts. Lady Newbie is a minor who communicates with Lord Bad Boy. Time to have a talk with her parents.
2) Lord Bad Boy is reportedly cheating on his spouse and is an overall jerk. Lord Newbie is not a minor but seems to enjoy Lord Bad Boy's company. I wouldn't touch it.

In your position of power, trust is even more important. I have no doubt the decision you'll make will be the right one for the situation. Anyway, I know you didn't specifically ask me but it seemed relevant to the topic at large. Thanks for sharing and good luck.