Shaykh al-Islām—may Allah have mercy on him—was asked:

A woman separated from her husband, and another man proposed to her during her waiting period and is supporting her financially. Is this permissible or not?

He replied:

Praise be to Allah. Explicitly proposing marriage to a woman during her waiting period is impermissible, even if she is observing the waiting period following her husband’s death, by the consensus of the Muslims. How much more so if her waiting period follows divorce! Whoever does this deserves a punishment that deters him and others like him. Both the suitor and the woman are to be punished, and he is to be deterred from marrying her, as a punishment that thwarts his intended purpose. And Allah knows best.

He was also asked:

A man divorced his wife three times. She completed her waiting period in his home and then left. After completing her waiting period, she married another man and was divorced that same day. Her former husband did not learn of this until the following day. May he agree with her that he will take her back in marriage once she has completed her waiting period?

He replied:

During her waiting period following divorce from another man, he may neither propose to her nor support her financially with a view to marrying her. If the divorce is revocable, he may not even hint at a proposal. If it is irrevocable, there is disagreement over whether hinting at a proposal is permissible.

This applies if she entered into a marriage out of a genuine desire for married life. If, however, she entered into a marriage intended to make her lawful again for her former husband:

“The Messenger of Allah, may Allah bless him and grant him peace, cursed the man who marries a woman to make her lawful again for her former husband, and the man for whose benefit this is done.”