Is there anything that can be done for a father-in-law who died 20 years ago and was not prayed over [the Janazah prayer]?

If a Muslim dies and is not prayed over, then it is obligatory to pray over him. This is done either by going to his grave and praying over him—this is specific to men—or by performing the Absentee Funeral Prayer (Salat al-Gha'ib) if he is outside the city, which includes both men and women. The funeral prayer is a communal obligation (fard kifayah). It has been established that it is permissible to pray over the deceased after burial, just as the Prophet, peace be upon him, prayed over the one who used to clean the mosque. It is also permissible to pray over an absent person, as he prayed over the Negus. Scholars have stipulated that if the deceased is buried without being prayed over, then prayer should be performed over him after burial, even if years have passed. There is no other act, such as charity or anything else, that can substitute for the funeral prayer.