All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Islam is a religion of cleanliness and hygiene so it deems it obligatory to remove impurities (i.e. Najaasah) and rendered purity a perquisite and condition for the validity of prayer. One should purify (material) impurities from his clothes and body by pouring water over them in order to get rid of all traces of the impurity as well as its properties (i.e. taste, color and scent).
However, in Islam, ease and avoiding hardship is well-established. Islam exempts people from purifying the (material) impurities that are difficult to remove so as to make things easy for them and relieve them of hardship. Muslim scholars have relied on this religious rule and believed that there is no harm if the color or scent of the impurity could not be removed after sincere efforts are exerted to remove its properties.
Imaam An-Nawawi said, "If it is difficult to remove the scent of an impurity, while its other properties were removed, then there are two opinions in this regard. First, the correct opinion, which is the opinion of the majority of Muslim scholars, is that if we considered this item to be pure, despite the scent or color of the impurity that could not be removed after purification, then it may be considered pure but it could also be considered impure. However, such an impurity is of an insignificant amount and may be overlooked; it does not render it impure."
Hence, after her diligence in removing the scent of the impurity from her hands, your mother is not obliged to wash them again and is also not obliged to repeat the ablution.Allaah Knows best.