I advised my friend to offer qada prayers of his deceased father and to pray for his forgivness as this is compulsory (wajib) on the children. He replied that his father had left him and his mother. He also said that his father didn't fulfill their needs and didn't pay attention to him and his mother. Because of that, he said it is not wajib upon him to pray for his father and offer his qada prayers.
What does Islam say about this?
You should pray for your father's forgiveness as it is one of the three things that can help him now.
The Messenger of Allah (PBUH) said:
When a man dies, his deeds come to an end except for three things: Sadaqah Jariyah (ceaseless charity); knowledge which is beneficial; or a virtuous descendant who prays for him (the deceased).
[Sahih Muslim]
Your praying will also make you a better person.
As for praying his missed prayer, that can not be compensated by you praying for him instead. For details see this answer.
A father who passes away it is obligatory for his oldest son (only oldest son alive at the time the father passes a way, even if the oldest son is 1 minute older than his twin) to do his Qadha Salah, no matter if the father was a good one or bad one. If the father was an atheist then I guess not but I am not sure. It is common Fatwa of the scholars that even if the father was Fasiq still the oldest son must do this. Some scholars even give Fatwa that doing the Qadha prayers of a deceases mother is also obligatory over her oldest son similarly.
If the oldest son refused to do this it will not be obligatory for the other children or grandchildren of the father (or mother), however, always anyone who likes can do this as well, the younger son, the daughter, the grandchildren, the nephews, the parents, the uncles, etc. and even the strangers. Even one accepts to do this it will not be obligatory over the oldest son any longer. However, the oldest son should ensure that the Qadha Salah will be performed. Even the oldest son can give some money to a person to perform this (and even to perform Qadha fasting which is equivalently obligatory for the oldest son) and likewise should ensure that the paid person will and is doing his task. That is, if the ones who have accepted to so such do not perform their task again the oldest son should do it, or find a way for others to do it.
If a mother only born a child and never ever meet her child till, say, the child is 40, and another woman grow the child to this age, the woman from whom the child has been born is still the child's mother and all responsibilities that a child has against its mother is preserved, even now that the child has never understood the kindness of his/her real mother. Likewise, the father is always a father for his children even if he does not do his duties or even was always angry with his family, teasing them and things like that.
PS. According to a scholar, if a father dies and he was Fasiq not noticing his Salah, yet the son should perform his Qadha Salah. This doesn't mean necessarily the father will have those Salah in his own record at the Day, but the son has done his duties right and a wisdom for such a rule can be that performing Salah on its own is a gift for the oldest son (like Nafilah of night being obligatory for only the holy prophet peace be upon him and his household was a gift for him) not an extra duty to tease the oldest son. You may like to see this story like a man was grown and treated while being a child by a Fasiq and now Allah wants him to pray more than the others to recover what he has probably lost, compared to the others. Note that this interpretation is not always true, it may be true that a believer gets sick at his last days and misses a number of his prayers at the hospital, then the Qadha Salah performed for him would of course be recorded in the man's own record, like himself has performed those Salah, especially if he wanted to perform but couldn't. Other special cases also exist that doesn't fit here.
