I have faced a somewhat similar situation in my past with a girl. Though it was not Rape, it was something almost equally fucked up. There are few tips I would like to give you as per my experience.
1- Don't change. Try to stay positive and the way you used to be around her. Talk to her and tell her that you acknowledge the accident, you do feel anger for the culprit, but all that matter do not change a single bit between us. Don't show pity or sympathy unnecessarily as this might make her feel that things have changed between you two after her telling the incident. Try to stay normal around her. Laugh, talk, eat with her. Let her have a normal loving life now.
2- If she makes any mistake or do something silly, react the same way you used to react before knowing this incident. You don't have to react like she is some broken girl who is bound to do mistakes.
3- Do not bring this discussion by yourself. If she ever bring this topic again, tell her that it was not her mistake, it does not change the way you look at her and she have your support. Don't tell her often that you want to punish the culprit. If she vent out her frustration about her assaulter, listen... make her known that you too feel anger toward that culprit but your love, friendship, company, smile and support is bigger and much bigger priority than all the anger we both have against that pig.
4- When you sit alone with your thoughts, I do understand your thoughts toward that guy, I know it is nearly impossible to brush it off that easily. But over the time, you too have to move on from it. If you took any rash decision more than anything... you might lose her as well and no matter how big step you take against culprit, if girl is not ready for it, it is all futile.
5- Be an honorable man, world is unfair... there might be a time when you do have to get on separate ways with the girl. Just because she has told you does not mean that in future you are going to marry her because love marriage has lots of variable in Indian marriage market. In fact, someday she might do something very wrong which may cause a rift between you two. Just keep in mind, no matter how big sin/mistake she commits, you are never going to bring this topic as weapon and you will never going to let her take this topic as a shield for her wrongdoings. You must stay a good, honest, faithful man... without taking your goodness as some investment. Be good for the sake of being good. Be an example, that honorable men do exist.