How can I convince my best friend not to have ?

Here's the deal: My best friend, Sally (which is not her real name for privacy purposes), wants to have with her friend, Chad (fake name). They used to be just friends. Chad got dumped by his girlfriend and he started to flirt with Sally. They would make out in secret and one day Sally finally told me what was going on. She also told Beth (fake name), Chad's ex-girlfriend. We're all really good friends. Beth said it was okay because she doesn't like Chad anymore. So Sally and Chad became friends who just make out with each other. Sally is moving far away from home at the end of the summer. She also doesn't get along well with her parents. She told me recently that she wants to have with Chad. We both have different opinions on premarital . I think it should wait until marriage, but isn't as big of a deal to her. I'm not trying to convince her to wait until marriage. I told her she should wait until she finds somebody she loves and then the experience will be more pionate. She said that one time they were making out and they got really far but none of them had their pants off, but she told me it was pionate. I asked her why she wanted to do this, and she said because she wants to. Chad and Sally don't even love each other. They aren't even dating. They just want to have . Sally is a whole year older than Chad, even though they're in the same grade. She said that since she doesn't get along with her parents and family, this makes her feel like she is actually being loved. I don't understand. Sex is not love. It's called love-making, but that's when you do it with someone you love and care about. How can I convince her that she should wait? She thinks she is ready! I tried to tell her that once she loses her virginity she can't get it back. She said "So?" I tried to tell her that when she does this, it will get easier and easier to keep doing it and once she moves, she might have sex there and she could spread diseases or catch one from a new boyfriend or sex partner or something. How can I put some sense in her to wait? Someone help me, I'm clueless. Also, don't forget that she's doing this with Beth's ex-boyfriend. And Sally, Beth, and I are all really close friends. Beth told me that she's upset that Sally made out with Chad and lied about it. She's also mad that Sally did it so soon after the break-up. Beth does NOT know that Sally and Chad are planning on having sex. And Sally told me NOT to tell Beth. How can I convince Sally not to do this yet?