How could she do this?

Both of them are obviously toxic . _ . I'm really sorry you had to find out this way. It'll hurt for a while. And you have every right to be angry and sad. But for your own sake, you should probably cut them out of your life. If they're going to walk all over you and treat you that way with absolutely no regards for your feelings, they are not worth your time. I hope you heal well. I'm just sorry it turned out like this.

She has every right to date whoever she wants. Just because she said she has no feelings for him doesn't mean she was telling the truth. Maybe she kept that from you to spare your feelings. The thing is, she's been given an opportunity and she's taken it. The sooner you learn not to be so bitter, the better. Just because she's your friend doesn't mean you have more right to him than she does.

As Your Friend She Should Have Respected you and Atleast Asked if You didnt Mind because She Knew You Liked Him. You Deserve Better in Both a Friend and A Boyfriend

Then she isn't really your friend and he is a pathetic asshole who are just perfect for each other. Knowing that he only want to use me for sex would be such a turn off. I'm sure you can be in love with someone who is 10x better! Your "best friend" can go fuck herself. What type of friend does that? That's just idiotic and malicious. She's obviously jealous of you. Why? I don't know. Unless the definition of friend has changed, um friends aren't supposed to be jealous of one another. Don't cry over this. They aren't worth your tears. You'll make yourself sick from crying. Don't even get upset either. It's their lost anyways! You should just cut them off and move on with your life. They're now in the past.

Yea okay, maybe she has always liked him to, but since you told her you liked him first she decided not to tell you anything, or maybe somewhere in the last three years she started to like him, but didn't say anything because again, she already know you like him -but she is your friend... your best friend at that.. so she should have known how much this would of hurt you and as a friend she should have given you a sort of heads up.. that is unless she prefers a crush over a friend.. Or she could've not hooked up with him. That would have been a cool friend to.
Honestly I would be angry and hurt. I would feel betrayed and I would want to rage like no other... but they are together now. I'm sorry that it's hurting you this much and I'm sorry a friend did this to you, but maybe now it is time to let these feelings for him go seeing just how much he doesn't care for you. Knowing how strongly you feel for him, but only wanting sex.. not letting you know he was into your friend so that you could back off.... he probably just liked the attention or like playing with you.
I don't know if I would be mad at your friend forever, because you and this dude weren't dating or having a relationship..unless you guys were sleeping with him in which case, yea she's messed up..but if you had no relationship with him than she didn't take anything from you really.
If she really did do it to prove she's better than you at everything than she just made herself look like the biggest bleh.. If she were better than you than she would not have done that because a better person wouldn't stoop to that level to make someone feel bad, their awesomeness would do it for them. If she were better than you she wouldn't be dating a guy she doesn't like, toying with his possible feelings for her. If she were better than you she wouldn't be talking you down to other mutual friends. If she were better than you she wouldn't lie. Personality makes a person.
Remember not all friends are true, but not all friends will be her. Sadly, you can only move on. Talk to her if you think it will make you feel better, but I wouldn't expect much from either of them. It might only make you feel worse. There is nothing you can do but let the pain ride. Hopefully you have other friends to turn to that will be there for you. This was just not right for a friend to do. As a friend, there was a better way to handle this. And if you are bitter, so what.. human emotion. It happens. It'll pass.

Well, if your best friend took advantage of your love for this guy who only uses you as a friend with benefits, I'd say you break your feelings for this guy. Then stay clear from your friend for a few days. She'll feel stupid that she hurt you like that, and once she has, that should be the time you can forgive her. Is this the first time she's done that, by the way? I'm only curious because I want to understand further the pain she's caused you at this point. My profile name is wise-nonexpert, if you ever want to visit my profile and need to talk about this.

I completely disagree with Lily. She shouldn't have lied to you. If she liked him then she should have said that and if she really is doing it just to prove she is better for whatever reason then she is a low life. You don't need that in your life. Cut ties with her and him and move on. The more you show how upset you are over this, the more power they have over you. I had a friend like her and she did the same thing to me. I stopped talking to her and him, refused to talk about the situation and just moved forward. Whatever you do, don't say bad things about them even if you feel them. Write it all down in a journal if you have to. My advice is just cut ties now. I know it hurts like hell but you will feel so much better when you move on.Try this: when you wake up in the morning or start feeling upset over this go to a mirror and tell yourself that you WILL get better. Tell yourself you don't need either of them, that you are a beautiful, smart young lady and will move on and be much happier.