Hi guys it's been a while... So my friend went through her first major high school break up a few months ago and now she's dating some guy she doesn't like. And the reason for her dating him is "Oh, you know. Just because life and whatever." Like....Um what? I'm really worried about her. She said she loved her last boyfriend and I told her she doesn't know what love is! We're teenagers! We don't know what that is. She insisted that she loved him. But he broke up with her via text and now she's changed, she doesn't care about finding some one special anymore. I was right btw that guy (We'll call him...Ass) didn't really care about her. Ass just used her over and over again. All she did was give. Give him her trust and affection. He never gave back. They had sex I didn't say anything about that. I chose to stay quite and let her ride this out. (No pun intended you pervs XD) So Ass moved somewhere else and I hope he rots. Anyways. Her sister told her "Well if you do it and you love him, and you two do break up you'll know you gave it up to some one you loved." Worst sister ever. She doesn't know about love she's a whore. So now I feel bad. I feel like I let her change into this darker person that just doesn't care anymore. Please help me. I don't know what to say.

...That's kind of scary.
Just don't leave her behind. Ya know, if possible make her face her last bf and have a proper breakup. This sound like a shojou manga but seriously, if she doesn't accept the fact that they broke up she'll ruin herself and not move on at all.
It pays to have a friend beside when in need. If you want to stop her, then stop her. Because if you let her go and let her do what she thinks she wants then shell think that "nobody cares" and just run around like that. Let her at least know what you think.
And you know, i kinda get you. The sister's a b*tch. Shes supposed to console her, not depress her.
I hope this helps

I don't think the sister did such a bad thing, she did try to console her, possibly not knowing that Ass was an ass.
I think you should seriously confront her with her ideals, in person. Point out what exactly she's doing right now and how wrong it is to betray herself. Tell her how you feel while watching her give up on life when you still believe she can get back up on her feet, open her eyes to all the possibilities that are still available to her.