I can't really tell you if it is a good idea or a bad idea. I don't know your friend. However, if she has been through a similar experience, I can't imagine that you would freak her out that much. I remember the first time I told a friend about what I endured as a child, and it was one of the best things I ever did. I gained a new best friend, and in turn he revealed that he had been abused. Telling each other our stories has helped us open up to other people, and even when someone does freak out about it (which has only happened once, and didn't by any means ruin the friendship) they generally get over it, or just don't talk about it, which is what you are doing now, so nothing much will change.

Really there is no great time to tell something like this, you've just got to go with your intuition.

Hope this helps, and please keep us updated on how it turns out.
*safehugs* (if ok with you)