How can one convince a person who believes the Umayyads distorted the Sunnah, and thus rejects it, otherwise?

The summary of the answer is:

Attacking the Sunnah and casting doubt on its preservation and documentation is an entry point for attacking the religion. Enemies of Islam, particularly the Orientalists, have been keen on this. They claimed that the documentation of the Sunnah was influenced by the orientations of the caliphs and kings after the Rightly Guided Caliphate, and that distortion and fabrication in hadith occurred for political and social purposes with the help of scholars and narrators. This false claim relies on invalid narrations or the distortion of authentic narrations from their true meanings. Among the most famous proponents of this specious argument is the Hungarian Orientalist "Ignác Goldziher". For a detailed answer, one can refer to Dr. Mustafa al-Siba'i's book "The Sunnah and its Place in Islamic Legislation".