All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is permissible for a divorcee to supplicate her Lord to make her ex-husband take her back, and we are not aware of any Islamic ruling that prevents one from doing so. That is because it is permissible for her to remarry her ex-husband, and it is permissible to supplicate for something to happen so long as it is permissible. Allaah Says (what means): {And your Lord says, "Call upon Me; I will respond to you.}[Quran 40:60].
Furthermore, it is confirmed that the Prophet said: "The person's supplication is answered unless he supplicates for a sin or the cutting of relations (i.e. with kinfolk)." [Muslim]
Nonetheless, it is more appropriate for a person to supplicate to Allaah to decree for him what is good and place the matter in the Hands of Allaah. Moreover, one should not rely on what he wishes as goodness may be in other things.