Every time an ex-boyfriend quarrels with his current girlfriend, he will come to me to complain. What should I do?

Actually, it's okay for your ex-boyfriend to complain about you. You coax him back to others, which shows that you are a quality person. You know it's not worth breaking up like this, and you don't want the tragedy to happen again. That's because you still have him in your heart. If you love someone, you want him to be happy. You don't have to intervene between them, but don't take love as your spiritual sustenance and let yourself get more stimulation.