Speaking from experience.........
I was in the position of the daughter, though not even close to being a teen. I can only give the following advice.......
Please continue to be supportive for your friend. She is going through something that NO mother should have to go through (been there, too). But, also tell her that she may fall apart when she is talking to you, but do NOT let her daughter see that too often.
To let her daughter see her cry, and know that she feels badly about what happens, is fine. But not on a full-time basis. She needs to be strong for her daughter, and let her know that she is not to blame for the terrible thing that has happened to her, that she is still just as good as anybody else, and that anyone who considers her "damaged goods" is not a friend, and really not worth her time. And, let her know that she is very loved.
If your friend cries every time she thinks about this, or every time her daughter needs to talk about it, the daughter could just clam up and not talk. She could hold in all the pain, rather than cause her Mom more pain, and this is very unhealthy. The only way this child will ever be able to come to terms with what happened, is if she is given the support she needs, and allowed to talk about it -in her own time, not forced to talk.
I pray for you all. This is a horrible thing to happen, and it takes a long time to cope, and learn to go on. But, hopefully, this young girl will come to realize that there is still good in the world, too.
Advice for the daughter, if you should get a chance to show this to her, with her Mom's permission,........
Do not let this ruin your life. You are the same person now, that you were before. Do not let someone else steal that from you. The one that attacked your body, cannot touch your soul. That is yours. The only way they can win, is if you let them. Please remain strong, and keep your head up. It's not easy, but show that you are still a strong person, and able to survive.
If you ever need to talk to someone, please know that I am only an e-mail away.