The Lord shall forgive her — i.e., she would not incur the guilt or punishment which would otherwise have been incurred by neglecting to fulfil the vow which she had made.
Related Commentaries of Numbers 30:5
Numbers 30:5
5 But if her father disallow her in the day that he heareth; not any of her vows, or of her bonds wherewith she hath bound her soul, shall stand: and the LORD shall forgive her, because her father disallowed her.