Is it preferable for the father to inform his nephew (mother's side) of what happened and ask for his forgiveness on behalf of my grandmother, and what can be done for the deceased grandmother?

Reconciling disputing parties is a great act of devotion. This reconciliation is particularly emphasized when it is between relatives. Accusing someone of sorcery is a grave matter that is not permissible without clear proof, because the default assumption is good faith towards a Muslim. Assuming the grandmother's sorcery against the man is proven, informing him of it could lead to corruption, and the intended outcome, which is his forgiveness of her, might not be achieved. Therefore, there is no harm in seeking another way, such as asking him to overlook the grandmother's actions and forgive her, reminding him of her rights over him, and of the immense reward for forgiving parents and relatives, without mentioning the details of what the grandmother did. It is also advised to frequently pray for the grandmother and give charity on her behalf.