Praise be to Allah.
If a woman breastfeeds an infant girl and becomes a foster mother (mother through breastfeeding) to her, then her husband becomes a foster father to the infant, so he becomes a mahram for her. He is what the fuqaha’ call the “owner of the milk,” that is, the husband of the wet-nurse who breastfed the infant.
It says in al-Mawsoo‘ah al-Fiqhiyyah (22/249):
The “owner of the milk” is the husband of the wet-nurse who produced milk as a result of becoming pregnant by him.
The girl who was breastfed by his wife becomes a mahram to the owner of the milk, because she is his foster daughter (daughter through breastfeeding). End quote.
We have noted previously, in the answer to question no. 146850 , that breastfeeding does not affect the siblings and direct ancestors of the child who is breastfed.
The direct ancestors are the father, mother, grandfathers and grandmothers.
The siblings are his brothers and sisters, and their children.
Therefore it is permissible for the father of the child who was breastfed by a woman to marry that woman (the foster mother of his child), according to scholarly consensus.
Al-Bahooti (may Allah have mercy on him) said: The wet-nurse is permissible [for marriage] for the father of the child whom she breastfed and it is permissible for her to marry that child’s brother through blood ties, according to scholarly consensus, and the mother of the child who was breastfed and his sister through blood ties are permissible [for marriage] to his foster father and foster brother, according to scholarly consensus." (Sharh Muntaha al-Iraadaat 3/214).
Based on that, it is permissible for the husband of the wet-nurse to marry the mother of the girl who was breastfed.
And Allah knows best.