I'm going to sleep with a guy my friend is interested in
That seems very bad. Why would you specifically sleep with that guy?
I'm not surprised, he is just a friend not a lover, that's why.
I get that that is not her lover, but you still know that she is interested in him. You should not sleep with someone you know that your friend is crushing on.
I know, but I'm going to sleep with him anyway.
Okay, then you should prepare with the retaliation from your friend and you may end up ruining your friendship over this, and you may really regret it in the end. You can get a lot of sexual partners in life, but friendships are hard to come by so I think that you purposefully ruining your friendship is a bad move.