Did I do the right thing by telling her?

Hello

I dated a guy for 2 years, he cheated on me, I broke up with him. We didnt speak for a year, and then we began speaking and we turned out sort of sex-buddies. We only did it a couple of times, no strings attached, maybe once a month, we didnt keep in touch in between etc.
We started seeing each other in march. On facebook he has the in a relationship status. I told him are you in a relationship? cause if so, I don't want to do this with you. He said he broke up with her long time ago and that he just kept the status because its nobodys business to see when and if he broke up and what hes doing.
Since then, we slept together 2 more times, although he was asking for me to come to his place maybe once a week, but I didnt feel like it.
Yesterday we spoke on facebook, he invited me today to his place. But yesterday night I found out from one of his guy friends that he knew we were sleeping together, although i told him not to tell anyone.
So I wrote him a message on facebook :did you tell X that we're sleeping together? Answer: you should ask him! me:what do you mean? answer:this is not Y(ex boyfriend name) me:then who is it? answer: I'm his girlfriend !!!!
My jaw dropped..
And then we started talking and she asked me is it really true that you slept together? They were together for a year or something, they lived together.
And I said yes..a couple of times. And i sent her the last messge he wrote me on facebook, telling me he wants to see me and all kind of sexual messages.
She was totally surprised, although she said he never let her touch his phone or computer.
I told her that that's what happened to me also and that she deserves better.
The next day he wrote me a message telling me why the hell did you do that? what do you have to earn? and then he asked me when did we sleep together? as if he couldnt remember. Then he called me a liar and started cursing me and so on.. I blocked him and never want to speak to him again. I'm sure he told her that I'm lying and that I'm crazy and that I want him back or something..although he was always the one to message me first and so on.. But from the conversation she can see that he wrote to come to his place and have sex.
My question is..did I do the right thing by telling her or that wasn't my business?
I felt like she has the right to know what kind of a person he is, I suffered when he did that to me(yes, I know I shoulve never sleep with him again, my big mistake) and she seemed a really nice girl..
Thank you!

Personally I think you did do the right thing. If someone was sleeping with my partner behind my back & she didn't know he was in a relationship I would want to know. It's not fair for her to be in a unfaithful relationship.

yes you did the right thing. the only wrong thing you did was getting back involved with the loser even though you knew what he was like a year ago. That was stupid and desperate. Learn a lesson from this. NO SECOND CHANCES especially not to cheats. Silly girl.

Yes you did the right thing.

I'm baffled as to why the hell you continued sleeping with that sleazy lying cheater though.

You did the right thing. He would have continued cheating anyway. It's better that she finds out sooner rather than later.

If I was in her position, I'd want to know and would be thankful that someone saved me from wasting my time with a douche. How she proceeds depends on her; if she believes his lies and goes into denial mode, that's her business.

Fair play to u for telling her, I'd definitely have respect for a girl that faced up to me if I were in that situation again.

I have a feeling that you contacted the GF directly, rather than her reading his FB messages...

Regardless, no, you should have kept out of their relationship. You have no idea what's between them, what their commitment is, etc. and you're right, he probably did tell her you were crazy and did that just to get him back.

The most important part of this whole story, however, is why you would become a FWB with a guy who cheated on you to begin with.