My wife had a dispute with my daughter-in-law and my son interfered, telling his wife, "I swear that you will be divorced today or tomorrow". He intended to threaten her due to her ill-mannered conduct with his mother.
What is the Islamic ruling on that?
If the case is as you mentioned, his saying to his wife that she will be divorced on that day or on the following day is not Talaq (divorce pronounced by the husband), rather it is a threat that he could divorce her soon. However, his oath that she will be unlawful for him requires making Kaffarah (expiation) for oath if he does not fulfill it by divorcing her soon. Kaffarh can be made by feeding ten needy persons the average of that which he feeds his family, half a Sa` (1 Sa`=3 kg. Approx.) of wheat, dried dates or rice or the like for each person, clothing them, or freeing a slave, and anyone who can not afford this should fast for three - preferably consecutive - days. Allah (Exalted be He) says: (Part No. 20; Page No. 178)
O Prophet! Why do you forbid (for yourself) that which Allâh has allowed to you, seeking to please your wives? And Allâh is Oft-Forgiving, Most Merciful.
Allâh has already ordained for you (O men) the absolution from your oaths.
And He (Exalted be He) says:
O you who believe! Make not unlawful the Tayyibât (all that is good as regards foods, things, deeds, beliefs, persons) which Allâh has made lawful to you, and transgress not. Verily, Allâh does not like the transgressors.
And eat of the things which Allâh has provided for you, lawful and good, and fear Allâh in Whom you believe.
Allâh will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation feed ten Masâkîn (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much).
In addition, this man should fear Allah and avoid making unlawful what Allah has made lawful. He must perform Tawbah (repentance to Allah) and seek Allah's Forgiveness, for Allah is All-Forgiving and Merciful. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.