What is the Islamic ruling on advising a friend not to buy from a merchant who cheats?

Warning Muslims against a deceitful merchant is permissible and not backbiting. Rather, it falls under the rubric of obligatory advice according to Islamic law, for "religion is advice (Nasiha): to Allah, to His Book, to His Messenger, to the leaders of the Muslims, and to their common folk." Jarir ibn Abdullah pledged allegiance to the Prophet, peace and blessings be upon him, to offer advice to every Muslim. Advice (Nasiha) means desiring the betterment of the advised person's situation and freeing them from fault and deceit.