All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It seems to us (Allah knows best) that it is permissible for a woman to supplicate that her husband does not marry a second wife, as there is no sin or aggression in it. A hadeeth reads, “The supplication of a slave continues to be granted as long as he does not supplicate for a sinful thing or for something that would cut off the ties of kinship.” [Muslim] Sheikh Abdul Kareem Al-Khudair was asked about this matter, and he answered that it is permissible.
Allah knows best.