The title seems bad but let me give context:
(This might get long, TL;DR at the bottom)

[SLIGHTLY NSFW]

I am a girl, and my best friend is a girl. We've been best friends for a little over a year, and when we started hanging out, I had a huge crush on her. (I'm gay and she's bisexual.) I'll call her Grace.

Since we used to have like a kind of flirty relationship, we've always had like a certain level of sexual tension between us.

About a month ago, she started dating this guy she used to be friends with. They've been "official" for about two weeks. I'll call him Edward.

So Grace and Edward have been going out for a little bit, but they still haven't had sex, because she had a medical procedure. Grace is generally a really sexual person so let's just say this has NOT been easy for her. 

Fast forward to today, the day of the incident. Grace and I always talk on the phone together when we can, and today was no different, however it's worth noting that today is the first day in over a month she's been allowed to masturbste or have sex or anything. We were talking as per usual, and I heard a weird noise, so I asked her what it was. It turned out it was her masturbating. (Like I said, she's been looking forward to being able to do this for a month.) Next thing I know, she's asking me if I'll masturbate with her. One thing lead to another, and to be brief, we had phone sex for about an hour. 

After we finished, I told her I felt kind of bad because it felt like I'd helped her cheat on Edward. She said she felt bad too, and I agreed not to tell him, because she still wants to date him. 

I still feel bad though, and I know I probably shouldn't have let that happen, and I don't know if I should tell Edward or not. So anyway, AITA?

TL;DR: I had phone sex with my bisexual best friend even though she has a boyfriend.