M24 I saw my innocent best friend f23 topless and sucking dick

We've been friends for years and recently she's got really hot, blonde with big breasts. She's very sweet and acts very innocent but last week I was at a party and she spent a lot of the night talking to a guy. After a while they both disappeared. I went to try and find her and opened the bedroom door to see her on her knees topless running her tongue over one of my other friends balls