How do I talk to a friend who has started experimenting with sex?

Question:

Good evening sir,

Sadly I write to you seeking some sort of advice as I am not only troubled but do not know how to handle this situation.

One of my friends throughout high school and now college (although she is closer to my girlfriend) just did something I absolutely cannot believe. Despite her lack of relationship with Christ, she was always so sweet and innocent. My girlfriend has been trying to bring her to Christ and had been making progress with her although she has not become a Christian yet. She was texting this fraternity guy who she had known had slept around with a lot of girls. Although they had hung out once before and he claimed he needed to take a "nap" at 1 am in her bed, she kicked him out. This time was different. At 3 am she drove to his house and they both performed oral sex on each other. I am just utterly shocked along with my girlfriend. The girl told my girlfriend this but the girl doesn't know that I know (my girlfriend informed me). I honestly wish I would not have found this out as I just cannot look at her the same way. She told my girlfriend after a lot of hesitation, but she eventually said that it was so much fun and she can't wait to do it again.

Now seeing as she is not a Christian I don't know what to do. Do we continue to hang out with her as if nothing happened? Does my girlfriend try to talk to her about Christ still or is hope lost until she comes around herself? I know the girl has a roommate who is a very bad influence (sleeps with many guys), so that may have played some part. I care about my friend very much and I don't want her to progress down the path she is headed. What does the Bible say about this matter?

Thank you for your time.