She is not considered unlawful for him, because
breastfeeding that affects marriageability has to take place during the first two years of the baby's life and the baby must be breast-fed for at least five times. This is based on the Ayah (Qur'anic verse) which reads:
The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling
The ruling is also based on the Hadith in which the Prophet (peace be upon him) said:
"There is no breastfeeding over the age of two years."
Other Hadiths have been narrated to the same effect.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.