The longer wording in Sahih Muslim continues:
لِلَّهِ وَلِكِتَابِهِ وَلِرَسُولِهِ وَلِأَئِمَّةِ الْمُسْلِمِينَ وَعَامَّتِهِمْ.
To Allah, His Book, His Messenger, the leaders of Muslims, and ordinary Muslims.
Sincerity toward Allah means exclusive devotion and obedience. Toward His Book and Messenger it includes honoring and following their guidance. Toward other people it means seeking their welfare, offering trustworthy guidance, and respecting legitimate rights.
If a friend makes a mistake, establish what is correct and then explain it gently. Agreeing with the mistake merely to please them, or humiliating them publicly while claiming to advise them, works against the purpose of sincere counsel. The goal of nasiha is genuine improvement, not victory or publicity.