My husband’s friend doesn’t believe in the sunnah and he has been misleading my husband too.
Those who reject the Sunnah are not Muslim.
Allah has instructed us to obey Him and to obey His messenger. If they don’t believe in the Sunnah, then they don’t believe in Islam or the Quran.
You have to convince them to revert to Islam which is composed of believing that Mohammed is Allah’s Messenger. Without the Sunnah, this would have no value to believe in.