Girls are retarded. I can't see how this is anything but a good thing. I have sex with my best male friend all the time.

Talk to her. The friendship isn't ruined AT ALL. Girls are stupid and petulant and emotional about sex. Ask her how she feels. Tell her you love her as a friend, and are attracted to her. Ask her what she wants for the future, and how she felt about the experience. If she didn't like it, then just apologize, remind her it takes two to tango, and that life moves on. JUST DON'T BE AWKWARD, and make sure you strongly advise her not to feel awkward about it.

Communication is best. Geez, it's only SEX.