In the gaming group I run, I said at the very first day that this was going to be a very "adult" campaign. I told them I wasn't going to pull punches, and that the party would have to really think about the decisions they made, as this was a gritty, dark world. In which true horrors and evil would be present. And they were all okay with that. But a few weeks ago I pushed it way too far.
One of the party members got himself kidnapped by a gang of criminals. (it was his fault, he was acting like an idiot.) So the rest of the party went to a rival gang of werewolves to ask if they knew where they could find him. Their leader agreed in exchange for the services of one female party member the following night. They freed the captured PC, and the female PC went to go honor the deal alone. When she went there, she was attacked by the leader. She was then assaulted, in a very bad way, by several of the gangs members. They let her go the next day, and the leader told the rest of the party that "She has paid for the information quite well."
Now, in my head at the time, I thought that this was a good idea. It created a villain the party had a real reason to want dead. With all the past villains the party had faced, they only fought them because there was treasure or someone else asked them. This gave them a person that they hated, a person that they all legitimately wanted to fight. And reminded them that it's probably a bad idea to go trusting violent gangs leaders. But afterwards, I realized that this was a terrible idea. I had never considered the human element. That there was a person playing this character that this "stuff" is happening to. I felt horrible about it afterwards, and apologized to the party, and especially to the female party member.
Now, the party member says that she's okay. That it did really remind her that she was dealing with criminals, and to make more careful decisions. But I still feel like I do owe her something for such a huge mistake. More so than just treasure or items. My group likes to go deep into RP, and was hoping to find an RP way to help the situation.
Even though the player said she was fine, it was very clear she was upset and depressed the rest of the session. And even a week later at the next session she was very insecure, and stayed quiet most of the time.
So I guess the real question is; What can I do to try and make up for such a terrible mistake?
EDIT: I wanted to thank everyone for all the wonderful answers you've given. Our next session is tomorrow, and we'll be going over what content is okay to show, and what people want to stay away from. Thanks to all the people who showed concern, and their own similar stories of experiences like this. You've all helped tremendously.