PLEASE PLEASE HELP. I NEED ANSWERS.?

[Nicole]

So I found out today that my friend has been giving head and having sex with my very close other friend. I know what you must be thinking, that he is fair game because we aren't dating, but you have to see it like I see it. I've known him since 7th grade, he's helping me through depression, he is 1 of the 3 people who know I self harm, and we both have feelings for each other. Then there's my other friend. Lately she's been acting like a slut, bragging about how many guys that want head from her. So she was on my phone on Facebook talking to some guy, I asked who was it and she didn't tell me anything. She gave my phone back and forgot to log out of Facebook. I went to log out for her but when I clicked the app the messages showed up. Of course I read them.
So here are some of what they said:
Girl
"You wanna hit it from the front and the back"
"Want me to give you head"
"I'm taking birth control"
Boy
"When we gonna met up"
"I don't have a condom"
"Do you have any diseases"

I'm as mad at him as I am at her. Also multiple people saw her making out with him the last day before Christmas break when I wasn't at school all week. Should I let this go? Should I kick her ass? Should I cut him off? So many options, I just want to make the right decision. I'm confronting her tomorrow about it.

So please help. What should I do? If you were in my position what would you feel like?