She slept with your friend because he is discrete. She slept with your friend because he is not needy for her to marry him before he has even slept with her or even having those kinds of conversations with her because those kinds of relationship conversations coming from a guy = total gayness to a girl. She slept with your friend because girls want and like to have sex just to have sex and they want things to develope naturally if they are going to develope past that, not feel like some guy who is talking to her wants to corner her into a relationship or is the type to judge her. She slept with your friend because he is not the type of guy who would bring up the other guys she potentially slept with before she has even slept with you.
Now
If your in a relationship with this girl and it is defined as a relationship and she is already humping you, then disregard the above advice or don't and remember it after she dumps you, which she is getting ready to do because your retarded. (a little harsh but I am trying to get through to you)
Finally
Let me give you an example of this girl I was talking to. She was interested but whenever I would bring up getting together or logistics she would get distant or nervous about it. I told her that obviously she doesnt know what she wants, and that I dont have time for it. I told her she was a cool girl and i definately liked her, but I only hang out with friends who know what they want because I am just too busy for that kind of sillyness.
She agreed to leave me alone.
Then she contacted me 3 days later. She asked me to call her. Then later when she had to get ahold of me again she practically begged for me to hang out with her, and she set up the time to get together. This is an example of a woman responding to a man WHO KNOWS WHAT HE WANTS. Who is a leader. Who has values. In fact one time I was talking to her on the phone and had her full attention. Some guy that was there ended up deciding to leave (not sure if it was a humpfriend of hers or not) I told her to tell him to stay because I am getting off the phone soon. That's called anti-needy. You should try it romeo.