What is the ruling on a married woman masturbating when her husband spends the night with one of his other wives?

Masturbation is forbidden (haram), and whoever engages in it must repent to Allah. We advise you to fear Allah (have taqwa), be patient, and remember the blessing of having a husband. Engaging in this habit is unbecoming for a married Muslim woman and may be a form of ingratitude for the blessing.