I have always heard from ulama that in the event one prays behind an imam who is a fasiq, the prayer of the best of those in the jama`ah replaces his as representative of the entire jama`ah. But of course praying fardh (obligatory) prayers in jama`ah, group prayer, is the way of the Prophet According to sahih hadith, praying `isha in jama`ah is the equivalent of praying half the night in prayer. So praying the night prayer in group is the best way even if there is a fasiq leading, but thereafter, tarawih prayer is not obligatory nor is praying behind a fasiq. Perhaps the shaykh was instructing listeners to avoid certain specific imams in the area who he addressed by this feature rather than by name. Allah knows best.