Nurse, this is what I gather from your post.
1. You want to do this, but perhaps with someone other than your friend.
2. It is a major fantasy of most men. Sure he wants to watch, but I think most men would watch while participating.
3. You want to know why, so ask him, if you go through with it, if you could have a 3-some with him and another male. Get his reaction. That should tell you if it's about sexual fantasy satifaction, or your friend.
4. What's your friend say about all this?