The idea behind women's "i am nor pretty" statements often maybe "I want MORE compliments from you, I want MORE of your attention to me, I want MORE of your nice word about me", regardless of how confident she is.
So you may give it to her

I disagree with jslaughter: you may say fake compliments as well, as soon as they are wonderful, cute, awesome or just nice. She'll know it is not the reality, but the reality is not what she wants.