Can a man marry his mother's sister-in-law if she is divorced?
Yes he can, any man can get married of any adult female who doesn't share a root or a branch with him. Neither the branches nor the roots of his wife (wife's daughter, mother, grand mother, grand daughter...) Special cases include your wife's sister, niece, and aunt, as you cannot be married to both at the same time, but it is possible after the divorce of the wife.
Under the Shari'ah, marriages between men and women standing in a certain relationship to one another are prohibited. These prohibited degrees are either of a permanent nature or a temporary. The permanently prohibited degrees of marriage are laid down in the Noble Quran:
"And marry not those women whom your fathers married, except what has already happened (of that nature) in the past. Lo! It was ever lewdness and abomination, and an evil way. Forbidden unto you are your mothers and your daughters, and your sisters and your father's sisters and your mother's sisters, and your brother's daughters and your sister's daughters, and your foster-mothers and your foster-sisters, and your mothers-in-law and your step-daughters who are under your mother-in-law and your step-daughters who are under your protection (born) of your women unto whom you have gone into -- but if you have not gone into them, then it is no sin for you (to marry their daughters) -- and the wives of your sons from your own loins, and that you should have two sisters together, except what has already happened (of that nature) in the past. Allah is ever-Forgiving, Merciful." [Noble Quran 4:22-24]
These conditions apply if the person affected is married. And if the person is divorced, she is not bound by the above mentioned conditions. As such, she is free to remarry