Assalamualaikum ustaz. What is the ruling for the aurah of a former mother in law and her son in law?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
The terminology mahram according to jurists means: “A person who is prohibited for us to marry with and the prohibition may be in the form of al-ta’bid (forever) for mubah reasons and not because of honour or there is something that prevents it that may be removed.” 
The explanation regarding the group of people who are considered mahram is stated in the statement of Allah SWT:
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ وَعَمَّاتُكُمْ وَخَالَاتُكُمْ وَبَنَاتُ الْأَخِ وَبَنَاتُ الْأُخْتِ وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ وَأُمَّهَاتُ نِسَائِكُمْ وَرَبَائِبُكُمُ اللَّاتِي فِي حُجُورِكُم مِّن نِّسَائِكُمُ اللَّاتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ تَكُونُوا دَخَلْتُم بِهِنَّ فَلَا جُنَاحَ عَلَيْكُمْ وَحَلَائِلُ أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلَابِكُمْ وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ إِنَّ اللَّهَ كَانَ غَفُورًا رَّحِيمًا
“Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.” 
The prohibition of marrying a certain woman is divided into two. The first is muabbad that is a person who is prohibited to be married forever. The second is tahrim muaqqat which is someone who is prohibited to be married to for a temporary time due to certain factors that prevent it. If it is still done, then it is included as a batil marriage and it is invalid.
According to the above question, when a man divorces his wife, the mahram relationship he has with his mother in law isn’t affected and they continue to be mahram forever (mu’abbad). This is based on the statement in surah al-Nisa’ verse 23, where it is stated that women who are prohibited to be married include:
“Your wives’ mothers,”
Syeikh Muhammad al-Zuhaili stated: And the prohibition remains even after death or divorce, solely with akad, even without intercourse ever happening between the husband and wife, for the verse only states mother to your wife, without the condition of intercourse. 
Thus, the limits of the aurah between a man and his former mother in law are the same as the ruling of between mahram which is between the navel and knees if it is safe from fitnah.
May Allah SWT grant us understanding in practising all HIs commandments and avoid all His prohibitions. Amin.
 See Kasyifah al-Saja, pg. 114
 Surah Al-Nisa’: 23
 See al-Mu’tamad fi al-Fiqh al-Syafie, 4/31-32