Is Your Ex-Husband's Father a Mahram?
Question

As a woman of Islam what should my relationship with my ex father-in-law be? Is it necessary for me to cover in his presence?
Praise be to Allah.Spousal Rights and Mahram Status Post-Divorce
The husband’s father is considered to be a woman’s Mahram even if her husband divorces her , because Allah says, stating who a one’s Mahrams are (interpretation of the meaning):
{…the wives of your sons who (spring) from your own loins.} [Al-Nisa’ 4:23]
In this case, the father-in-law becomes a Mahram  simply as soon as the marriage contract is completed; if a man makes a marriage contract with a woman, then the man’s father becomes a Mahram for his son’s wife, even if the marriage is not consummated.
This is what the scholars called Al-Maharim bil-Musaharah (Mahrams by marriage).
Categories of Women Who Become Mahram by Marriage
The women who become Mahrams through marriage are of four types:

Women married by one’s father (i.e., father’s wife and also grandfathers’ wives). Allah says (interpretation of the meaning): {And marry not women whom your fathers married.} [An-Nisa’ 4:22]
One’s wife’s mother or grandmother: {Forbidden to you (for marriage) are… your wives’ mothers…} [An-Nisa’ 4:23 – interpretation of the meaning]
One’s stepdaughter (a wife’s daughter by a previous husband). The stepdaughter is not a Mahram unless the man has consummated the marriage with her mother. If he simply made a marriage contract with her but did not consummate the marriage, then she is not a Mahram, because Allah says (interpretation of the meaning): {your stepdaughters under your guardianship, born of your wives to whom you have gone in — but there is no sin on you if you have not gone in them.} [An-Nisa’ 4:23 – interpretation of the meaning]
One’s son’s wife and one’s grandsons’ wives, because Allah says (interpretation of the meaning): {the wives of your sons who (spring) from your own loins.} [An-Nisa’ 4:23 – interpretation of the meaning] (End quote from Jami` Ahkam An-Nisa’ by Al-`Adawi, 5/302)

Islamic Scholars’ Insights on Mahram Relations
Shaykh Muhammad ibn `Uthaymin (may Allah have mercy on him) said:
Allah says (interpretation of the meaning):
{… your wives’ mothers, your stepdaughters under your guardianship, born of your wives to whom you have gone in — but there is no sin on you if you have not gone in them (to marry their daughters), — the wives of your sons who (spring) from your own loins …} [An-Nisa’ 4:23]
These three are Mahrams by marriage. The phrase “your wives’ mothers” means that it is prohibited for a man to marry the mother or grandmother of his wife, no matter how far the line of ascent reaches (i.e., great-grandmother, etc), whether that is through the mother’s line or the father’s. She becomes his Mahram as soon as the marriage contract is done.
If a man makes a marriage contract with a woman, it becomes haram for him to marry her mother and she becomes one of his Mahrams even if the marriage with her daughter is not consummated. If it so happens that the daughter dies or he divorces her, then he is still a Mahram for her mother. If it so happens that consummation of the marriage is delayed, then he is still a Mahram to her mother; she may uncover her face in front of him and he may travel with her and be alone with her, and there is no sin on him, because the mother and grandmother of the woman become Mahrams as soon as the marriage contract is done, because Allah says “your wives’ mothers”, and a woman becomes a man’s wife as soon as the marriage contract is done.
The phrase “the wives of your sons who (spring) from your own loins” means that it becomes haram for a man to marry the wife of his son or grandson, no matter how far the line of descent extends, as soon as the marriage contract is done. The wife of one’s grandson becomes a Mahram to the grandfather as soon as the marriage contract is done. Hence if a man makes a valid marriage contract with a woman, then he divorces her immediately thereafter, she becomes a Mahram for his father and grandfather, no matter how far the line of ascent reaches, because of the general meaning of the phrase, “the wives of your sons who (spring) from your own loins”. The woman becomes permissible for her husband as soon as the marriage contract is done. (End quote from Al-Fatawa Al-Jami`ah lil-Mar’ah Al-Muslimah, 2/591)
And Allah knows best.