Yes it is permissible for husband's father to marry the mother-in-law of his son if she is widowed or divorced. Wife is Mahram for her father-in-law only not for his sons and husband is Mahram for his mother-in-law only not for her other daughters. This rule of being Mahram does not include the parents of the husband and wife.
'Your mother-in-law must observe Hijab from your father but not from you. Obviously, when they are non-Mahram to each other, marriage between them is permissible.