It is indeed permissible in Shariah for an infant to drink the milk of any woman; and the suck of an infant will develop a lasting foster and ‘mehram’ relationship with that woman.
Al-Muwatta Hadith 30.4
Yahya related from Malik from Thawr ibn Zayd ad-Dili that Abdullah ibn Abbas (r.a.) said, "The milk which a child under two years old sucks, even if it is only one suck, makes the foster relatives ‘haram’ (sacred)."
If the husband happens to taste the milk from his wife’s breast accidentally or voluntary, there is absolutely no harm and does not effect their ‘nikaah’ or marital relationship in the least.
Sahih Al-Bukhari Hadith 3.815 Narrated by Aisha
Once the Prophet (saws) came to me while a man was in my house. He (saws) said, "O 'Aisha! Who is this (man)?" I replied, "My foster brother." He (saws) said, "O 'Aisha! Be sure about your foster brothers, as fostership is only valid if it takes place in the suckling period (before two years of age)."
In light of the above clear guidance of the Messenger of Allah (saws), the consumption of a women’s milk will only bring into effect foster-relationships if the suckling baby is in the suckling period, which is unanimously accepted in Islam as being under two years of age. Thus the majority of the good scholars in Islam are of the opinion that if an adult accidentally happens to taste the milk of a woman, it does not develop a foster or ‘mehram’ relationship with that woman.