Need help telling my best friend I slept with one of her best friends

so about 9 months ago my best friend was telling me about some people she went on a camp with. I thought one of the guys was pretty cute so I added his Snapchat. My best friend sees this boy as a brother figure (even though they aren’t blood related). I ended up talking with this boy a lot, which my best friend was aware of. However me and the boy decided to meet up behind my best friends back, and we ended up sleeping together. We still talk a lot, and now, 6 months later, my best friend said she wants to take me with her to camp this year to meet the boy. I want to tell her we’ve already met before the camp starts. Does she have a right to be mad at me? How should I tell her?

I think if she has something to be mad at, it would that you went to see him without telling her. Remember that she's your best friend. You should be able to confide in her. Sit down with her and let her know that you have something to confess. Then tell her that you guys met up before and things kinda escalated. Even if she gets upset, I'm sure she'll come around.