1) If a married lady made a vow which involves physical distress, her husband may annul her vow by saying three times to her “Mufar Lach!”. This is only true on condition that he annuls it on the day that he finds out about it. This means that if he finds about it at night, he may annul it during that night and the following day. If he found out in the middle of the day, he may only annul it until the end of that day. (When one is discussing a married lady, there are other vows which the husband has the ability to annul, and each case should be asked to a competent wise man.) (Kitzur Shulchan Aruch 67).
2) On Shabbos the husband or father may not explicitly annul the vow. However, since once Shabbos has passed he will be unable to annul it, he should say on Shabbos expressions which implicitly mean that he wants to annul the vow. For example, if she vowed not to eat a certain food, he should tell her to eat it. (ibid).