What should a person do with a friend who has admitted to harboring malice towards him, and still feels this malice whenever he achieves any success, and what does Islamic law command in this situation?

The fundamental principle is to think well of Muslims and to interpret their words and actions in the best possible light. Umar ibn al-Khattab said: "Do not think ill of a word uttered by a Muslim if you can find a good interpretation for it." If malice appears from someone, do not fear, for benefit and harm are in the hand of Allah. Malice is treated with kindness and good words. Allah Almighty says: "And not equal are the good deed and the bad. Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend." Al-Ghazali mentioned that courtesy breaks animosity and accustoms hearts to harmony.