All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Islam urges the Muslims to be affectionate and friendly with one another and forbids them from cutting ties and deserting each other without a sound reason. For more benefit, please refer to Fatwa 86090 and Fatwa 84976.
Indeed, you did well by trying to reconcile between your friends because reconciling between two disputing parties is one of the greatest acts by which one gets closer to Allaah.
As regards the oath that your friend made, it is a void oath because it is not permissible to make an oath to other than Allaah, and whoever makes such an oath and breaks it, there is no specific expiation for it, but he has to repent to Allaah (from swearing by other than Allaah). For more benefit, please refer to Fatwa 82456.
If we presume that it is a determined oath because of it being made to Allaah or by one of His Names or Attributes, then the best thing for your friend to do is to urge himself to meet his friend and reconcile with him; for more benefit, please refer to Fatwa 81387.
Finally, we advise both your friends to reconcile and close the door of the devil as he is keen on separating between friends and lovers, and the best one of them is the one who starts greeting the other with Salaam.
Allaah Knows best.