When it's someone far off it seems less 'threatening' so to speak that they are more attractive but this girl is a friend of his whom he used to be closely intimate with and still wants to be. So it's a little more close to home.
Yes. It's always unnerving when it is close to home.
There's also dealing with the fear of "Ack! What if they leave me for them because they are better?!"
But really? They could leave any time. With or without another person in the picture.
So it isn't so much about where they go. It's what keeps them coming BACK -- be the best self you can be since it is YOU they want to be with and the unique YOU things you can provide.
Hang in there!