Put Somewhere Between Topaz and Corundum

Related to the same sixteen year old girl as my previous thread. (http://www.psychforums.com/relationship/topic120252.html)

Tonight we went out to dinner after tutoring with our mutual female friend (21). Food was good, and we planned to catch a movie. On the way between two locations, a random hipster guy came up and started talking to the girls. He mentioned secret dates out loud in front of me. I inquired. Everything blew up for a bit.

When we got home, the three of us picked through the topic. Apparently the sixteen year old girl had made secret plans to see her internet boyfriend mentioned in the previous thread on this past weekend. They were going to go out with her, the boy, and the boy's ex-girlfriend who was the whole reason they met. The random guy we ran into that spilled the story found out from the ex-girlfriend because she's a bit of a gossip and borderline and instigated some drama.

Plans had been cancelled because the ex-girlfriend couldn't do it. Both the little girl (16) and the internet boyfriend (23) say they didn't really want to do it anyway... but I'm not sure how significant that really is here.

It's also slightly annoying that the girl was going to use me for a ride to the ex-girlfriends, and she had intended on staying the night. And I'm plagued with the fact that the boyfriend guy never cops to or admits to doing anything wrong. He always has some weaselly excuse. And I see it for what it is, but I'm afraid the sixteen year old just believes him.

Now, my rock and hard place problem is my responsibility to tell the girls mother vs the damage that will cause in her family and the fact that I'm making progress with her on tutoring and increasing her self esteem. I specifically didn't want to be put into a position of power where I'm holding something (telling her mom) over her head. The outcome I want to see is her learn from this experience (it's actually the second set of secret plans she has had in this regard, and her mother knows about the first). I want to see her realize how much of a creep the guy is and move on... but he keeps digging her a deeper hole with every crazy event.

I will also say that I'm very glad things came up this way, because it would have been far worse if I had caught them in the act (which is likely, as I'm perceptive relative to everyone involved and already suspected something was up with the ride-request). Better an emotional blow out than whatever might have happened.