The two fell out over business and became enemies.
Tips
usage
Often expanded to the set phrase 反目成仇 ('fall out and turn into enemies'). The most common context is husband-and-wife discord. Also used for friends, siblings, and former allies.
history
反 ('turn back / reverse') + 目 ('eye') - to turn your gaze against someone. The phrase traces to classical usage where 目 represented the look of affection; reversing it meant hostility.
