You need to remind him not to make such comment

 

in front of her.  She should be the most beautiful girl in the world in his eyes. 

His comment chips away her self-confidence and teaches her what is and is not important for a girl to possess.  It also diminishes his standing in her mind when she becomes knowing enough.

Self-confidence is an attractive trait, more so than facial beauty.  A girl's self-confidence is heavily influenced by the role her father played when she was young.