Most Helpful Guy
Either he is doing something that he doesn't want you to know about OR he is not doing anything wrong but afraid that you won't believe that.
Most Helpful Girl
I've been there unless in my case, I was that "girl best friend who happened to be an ex too". What I believe is there is no such thing as a friendship between boys and girls. In my case, I didn't have feeling other than a lunch companion. At the end of the lunch I told him to tell his girl a "Hi!" from me. He said that he will.
And that's it. Nothing happened.
If your boy chose to lie instead, then he's ready to pay that sin.