What is the ruling on someone who wrongs another person, then apologizes and is forgiven by the wronged person, but the latter revokes their forgiveness after a period of time?

There should be no hesitation in accepting an apology, for accepting apologies and forgiveness are characteristics of noble people. Once the person with the right relinquishes their right, there is no going back. Therefore, your friend's retracting his forgiveness does not harm you, and there is no difference in this regard between someone you still meet and others.