How can one convince a friend who believes only in the Quran and not in the Prophetic Hadiths about the reality of the Sunnah, knowing that he considers Hadiths to be human sayings whose authenticity cannot be confirmed?

The Sunnah (definition) is a revelation from Allah, and its preservation is guaranteed by His preservation of the Dhikr, as it is part of it. The Ummah has unanimously agreed upon the authenticity and acceptance of much of it, and their consensus is a decisive proof that cannot contain error. Furthermore, it details acts of worship and transactions, and it foretells unseen events that occurred later. Therefore, whoever claims its lack of authenticity has confined the Prophet's message and mission to only those who saw him and heard from him directly.