Disclaimer: Not delving into the rules themselves as this feels more than a few steps out of what the authors had ever imagined.
Start with the question of how this God would inflict forgiveness on a feeling and comprehending being. Snapping their fingers and voila would be much to vulgar for a god. Instead perhaps the God would fulfill this wish by forcing awareness of the Revenant's creator, in this player's malicious or unintentional crime. Why did they do this, how did this person reach the moment of the crime.
What I am getting at here is that evil and wickedness don't just appear out of thin air, hate creates hate. Going backwards, eventually the source of pain will become too abstract and intangible to focus on.
So, perhaps this Revenant realizes it is futile to hate and enact revenge on their murderer. Those strong feelings don't go away so perhaps a solution would be to forgive the person as a whole and focus on murdering the parts of its killer that led to the Revenant's death.
Now the fun part of this and I hope one you will enjoy. God's can be twisted things, endowed with infinite power which makes their existence boring. Toying with the stupid things mortal's get up to is a reliable source of entertainment. By gifting enlightenment to the Revenant, helping it forgive its murderer, changing the nature of itself changes the focus of its wrath. Instead of trying to enact revenge, now the Revenant is focussed on fixing the bigger problem.
To put it simply: the Revenant becomes a powerful guardian spirit that will not assist the player in their evil ways but instead be the players guilty conscious. Every time they steal, murder, rape, or whatever evil act the player wishes to commit they will have to roll the dice for concentration (to overcome their Guardian's guilt). Inversely! Every time the player commits a good act they would have similar boosts of a paladin.
I leave it to you to figure out how to make this D&D rules legal but given how specific and bizarre this is I would recommend writing out rules, making them fair, but don't forget the sinister angle of this. Your player tried to cheat in a clever way and your wish granting god should be aware of that.