Yes, you can forgive someone who is deceased. Forgiveness includes thoughts, feelings, and behaviors. One can think of the other person as possessing inherent (unconditional) worth. One can cultivate feelings of compassion for the person, not because of what he or she did, but in spite of this. Even behaviors can be a part of the forgiveness. For example, one might donate to the deceased person’s favorite charity. One might say a kind word about the deceased to family members. Depending on one’s religious beliefs, the forgiver can offer a prayer for the one who died.
For additional information, see: Forgiveness Defined.