I went through something that sounds similar to this a while back (though, your description is vague, so I am just guessing). My advice:
I think it is important not to impose one's own perception of a situation. Simply engage in conversation about what happened and how it has affected that person, and let them come to whatever conclusions are appropriate for them. They may not be comfortable with calling something rape or assault, but they can still recognize that what was done wasn't okay and was harmful. Validate those feelings, don't force your friend to label them with your own words and ideas. In my experience that involved a lot of watching crappy television shows and cooking grand meals and being there for whenever the tears came. Just being a shoulder to cry on and a safe person to talk to when she wanted or needed and someone who knew her secret but didn't define her by it was more helpful than sitting her down and trying to work through anything intentionally or trying to convince her that I understood what happened to her in words she didn't/wouldn't relate to.
Also, be mindful of how your own experiences may cloud your perception. Your experiences may be relevant and helpful to this situation, but it may be harmful as well. Be there for your friend and support them in what ever way they need, but also find a support person for yourself that can be there for you in processing your experiences, the friend who you are trying to help is not going to be a good person to support you and trying to work this out with them is likely to be detrimental to their own processing of their situation.
Hope that helps, and best of luck.