The links, especially the one on the emotional aspects, will help you better understand what she's feeling and how to support her. It's hard to know whether or not this was a deliberate act. Men and women pass on STDs all the time out of ignorance and for other reasons. What's important is that she focus on her own needs. Sometimes just listening when a person needs to vent is a wonderful gift and accomplishes a lot even if it may feel like you're not doing anything. A lot of people have no one to talk to at all, and having a good friend like you is priceless.
Personally, I have a chronic form of leukemia and do know what it's like to learn you have a long term disease and your life has been turned upside down. My best friends spent a lot of time listening and taking the cues on what I needed from me as I went through the sorting out process and acquiring information. You are doing the right things. It might help you to understand the stages of grief people experience when they face transition. While the principles were originally applied to terminal illness it's now recognized that they apply to all major transitions and that people bounce back and forth between the stages before they reach the stage of acceptance. http://changingminds.org/disciplines/change_management/kubler_ross/kubler_ross.htm
Your friend is grieving for what she had before she got the diagnosis. We don't really prepare people for dealing with these things. The fact that you care enough to ask how to help is remarkable. This will get better, but it will take time. Hang in there.