How can one atone for wronging a deceased friend, so as to clear one's conscience before Allah on the Day of Judgment?

You must return the injustice you feel towards your deceased friend to his heirs if the right is material. If it is non-material, you must repent by regretting and resolving not to return to it. If you intended to fulfill the right but were unable to do so due to his death, it is hoped that you fall under the saying of the Prophet, peace and blessings be upon him: "Whoever takes the money of people with the intention of repaying it, Allah will repay it for him; and whoever takes it with the intention of destroying it, Allah will destroy him." It is recommended to supplicate for him frequently and give charity on his behalf.