Claire had her pride too.
She didn't want to be labeled the girl who married rich or the icon of a modern-day Cinderella story—instead of being a vassal to an important family, she'd rather have a boring, easy life.
Moreover, Quinn suited Charlie more than herself, and she definitely sacrificed a lot to find him.
Whether it was sentiment or rationality, Quinn deserved her blessing and respect.
She certainly could give the former by ending her marriage with Charlie, which counted as a virtue.