A guest in Lord Mengchang's household fell in love with one of his concubines. Someone urged Mengchang, "This guest serves you yet covets your wife—such disloyalty deserves death." Mengchang replied, "To be drawn to beauty is only human nature," and spared the man. Later, Mengchang prepared a carriage, horses, and gifts, recommending him to the ruler of Wei. Before his departure, Mengchang said, "We have been friends as equals; I hope you find great success serving the ruler of Wei." Thanks to Mengchang's recommendation, the guest was highly valued upon arriving in Wei.
Later, when relations between Qi and Wei deteriorated and the Wei ruler planned to rally allied forces to attack Qi, a retainer stepped forward to stop him, declaring, "Lord Mengchang is a man of great virtue. I wronged him, yet he did not punish me and even praised me before you. Moreover, I have heard that the former rulers of Qi and Wei once swore a blood oath that their descendants would never attack each other—now you are breaking that covenant. I beg you not to attack Qi; otherwise, I would rather spill my blood upon your robe!"
Thanks to his retainers' desperate pleas, the lord of Wei abandoned his plan to attack Qi.
"Bu Yi" refers to the clothing worn by commoners, and by extension, common people themselves. The idiom "Friendship of Commoners" originally meant nobles associating with commoners, and later came to refer to friendships among the poor and humble.
Source: *Strategies of the Warring States*, Chapter "Strategies of Qi III"
Meaning of the Idiom: Later, the Chinese idiom "布衣之交" came to describe Friendship of Commoners originally meant nobles associating with commoners, and later came to refer to friendships among the poor and humble.