The kind of person who calls themselves a Nice Guy and complains about Arseholes is usually the type who would call a girl beautiful when he doesn't really mean it and then complain that he is so Nice but never gets anywhere aka "Finishes Last".
The more likely truth is the girls bullshit meter has gone off and she knows he's not really a Nice Guy at all. The Arsehole however might not call her beautiful but she knows when he does say something about her, it's going to be genuine so she would appreciate that more.
If you were one of the false Nice Guys then you would probably know exactly what I meant without having to ask.
"You're marionette work is impeccable"