I get a call from my "best friend" telling me that she is sleeping with my ex. Now mind you, this ex is from 25 years ago and I have absolutely zero feelings for him and couldn't care less who he is sleeping with. The problem is she decided to tell me only after he told one of our other mutual friends in fear that I would hear it from them instead of her.  I have been avoiding talking to her for about a month now because I'm just not sure how I feel about it. Well that's not entirely true, I feel manipulated and I think that the way she approached me about it was disingenuous

I told her how I feel morning and she responds with this long text: 

"I didn't plan on saying anything to anyone really, we were just friends having sex. He told me about his major love of you from long ago. Then he tells me he told J about us hanging out (and I assumed he would talk about the sex). That's when I thought I needed to let you know before you heard from someone other than me. It was out of respect to and of you. 
I didn't think you were into him, just wanted as your best friend, to let you know first, that I am enjoying sex with your ex. I didn't want this getting out at all- he's going through a divorce and it doesn't need to be known by anyone that I'm a sex therapist in this situation! 
I love you and wanted to make sure you knew from me. Call if you wanna talk more- I can't type all the rest"