I know about a rape and I'm not reporting it
Why wouldn't you report rape if you know about it?
I don't want to ruin someone's life, the rapist was a good friend of mine.
If you want to protect a friend then it is understandable to a degree, but he committed a horrific act and raped a person. That itself is criminal and he needs to be punished. Report him without telling who you are, then no one can find out about you being a tattletale.
I have the intention to report, don't worry.
I'm glad that you changed your mind. It's important that justice be served in this situation.