Writing a monologue from Chris Keller's point of view after his father's death, we will have to use what we know about his character to imagine what he might have - hypothetically - have wanted to say to his father or about his father at his grave.
We cannot know if Chris would forgive Joe Keller after his death. It is possible that Chris would have blamed his father and derided his evasion of punishment by taking "the coward's way out". However, it is possible that Chris would have realized how extreme his own feelings had become and how he pushed his father to an extremely painful realization of guilt and responsibility (especially in light of Larry's death).
We can be sure that forgiveness (or the choice not to forgive) would be mentioned in a monologue in this setting.
We might also imagine that Chris would tell Joe that he has married Ann (or chosen not to marry Ann) and tell Joe how happy or unhappy he is as a result. Chris would be likely to also mention how his mother is doing without Joe, whether or not she has gained or lost strength after his death, and how she is doing emotionally.