BE SURE TO READ THE LAST PART OF THIS POST!
I have seen a similar situation to this. I was 17 when it happened as well and this was a fully grown man doing these things to his step-daughter, who I went to school with. I can't exactly remember his age- mid 30's if i had to guess. I'm 23 now. But anyways, I didn't play any direct role in this. I was just in a group of people who gave a damn about what happened in this town.
There were 6 of us that always hung out together and sort of played some weird "vigilante neighborhood watch". Basically, if someone was being a complete jerk or just acted like an unpleasant human being some or all of us would find a way to mess with them. No physical contact, no details, no traceable evidence. Most of the time we didn't know the people we targeted. But we confirmed the stories before we took action. I'll save any examples, except the one pertaining to the story I previously stated and this is how it went down.
My friends mom and her new husband were actually together for 3 years before they got married. My friend was 12 when they first got together. After they got married I noticed that (lets call her Jenny, not by her real name. Using the word she over and over would annoy me) Jenny was looking very flushed. Always red in the face and I didn't know why. I tried to talk to her about it, but all Jenny would say is that she's going "through that change". I believed it for a while. About a year later She was afraid of everything. She wouldn't hug anyone, she would get spooked by random noises. If someone brushed against her on accident she'd jump and freak out.
One of my friends came up and asked me what was going on. I said I didn't know, she wouldn't talk to me about it. They actually thought I was the one troubling her. (Lets call this guy Steve) Steve, the appointed head of the group talked to her and she confessed to him, but not everything. She told him that her step-dad was forcing her to do drug exchanges for him and got her high and drunk all the time. Later we found out that he was taking advantage of her while she was blitzed and passed out.
Well, this guy would do more graphic things to her, but I don't want to go into details. This "man" was pretty loaded. He had money to waste, which means he had things to lose. I played no part in these events as I had taken a trip to Bosnia for 3 months to visit my brother for his wedding.
One day his tires might be slashed. The next a broken car window. Then Steve got sloppy. He wrote a note. It said, "Leave her alone or things will only get worse for you." This is what got him caught and arrested, but we'll get to that part later. Lets call the molester Jim. Jim was angry that someone found out about his deeds and he upped his game. He started to punish Jenny and sodomize her. Steve had his fill. After 2 years of pretend, he got real. He had him and 2 other people stake out Jim's place until he was alone. Not sure on the time of day here, but sometime in the afternoon apparently. Anyways, they went into the house by surprise. Steve pinned Jim down while the other 2 beat him. That night the police traced the not back to Steve, and he was arrested for conspiracy to commit murder, breaking and entering, assault, and attempted murder. Steve was 18 and just about to graduate high-school. Now he's living off a 25 year sentence in prison.
After that Jim also got arrested for child abuse, drug trafficking, and domestic abuse. Facing a 15 year sentence and is a registered sex offender. Jenny was never the same 12-year old girl I used to know and play with again. She's now clean from drug and alcohol addiction and works a regular job. I know it sounds like she's well, but she never smiles. Ever. She fears what might happen after Jim gets out of jail. Some people may not understand, but these things are traumatic. They can have a life-long lasting effect. I still try to talk with Jenny regularly, but her mother doesn't like it because she thinks I'm a hooligan. I visit her at work or meet her for a coffee and we talk for a few hours. She doesn't blame me for anything. She keeps making that clear. I wasn't there to make any calls or have any say in what happened.
The moral of this story is to NOT take the law into your own hands. Don't try to play the hero. Nothing will come of it. You'll just get yourself in trouble. Alert the authorities. Give the most detailed information you can- Names, addresses, locations, actions, drug or weapon involvement, are weapons/violence involved, etc...