You can send mettā to a person no longer in this world, one to whom you felt resentment and who had resentment toward you.
Question: Is it valid to send mettā to a person no longer in this world, one to whom I felt resentment and who had resentment toward me? Thanissaro Bhikkhu: This is a very good practice to do. It’s one way of bringing your own mind to some peace. You have to remember that when people die, they don’t go out of existence. They get born again. So, they’re always there someplace for you to spread mettā to. Now, whether that person rejoices in your mettā or not, that’s that person’s business. But if you can spread goodwill to someone you used to resent, that takes a huge burden off of your mind. ~ Good Heart, Good Mind: The Practice of the Ten Perfections