J.D.
As the parents of teenagers this is a tough one. The fact that your child trusts you is a biggie and I would not do anything to betray that trust. Ask your child if you can talk to the parents, and if she say's no encourage her to get the other child to talk to you. You might be able to get her to. If she won't at the least you can help her find appropriate support. Perhaps the school can intervene? If they do it's crucial your child be kept out of it.
I'm sure alot of people will disagree and say you should talk to the parents. This child is 18ish, and if she does not want to their she just won't. Frankly, she may have a legitimate reason. Your relationship with your daughter is paramount. Good luck