Answered by Shaykh Amjad Rasheed
My wife joins her prayers because she is breastfeeding. What are the neccesary conditions to validly join prayers?
Nursing an infant is not a valid reason for joining prayers, for it is not permissible to join two prayers except for travel or rain.
Some of our Imams, such as Imam al-Nawawi, chose the position that joining prayers is permissible for the sick person for whom performing each prayer at its proper time would pose a hardship.
Such a person may pray [h: asr or isha] early [h: during the time of zuhr or maghrib] or [h: zuhr or maghrib] late [h: during the time of asr or isha]whichever is easier. If one joins two prayers in the time of the first of them, it is obligatory for one to not separate the two prayers by pausing between them.
Thus, if the nursing wife in question is in a hardship like the hardship of sickness, such that praying each prayer at its proper time is difficult, then she may follow the opinion of those who say that the sick person may join prayers as mentioned above.