Is it necessary to advise someone who openly and repeatedly engages in collective dhikr and du'a after prayer, or is there flexibility in the matter? And was this reported from the Salaf?

The arrangement of dhikr (remembrance of Allah) by the Imam, Muezzin, and congregants in the manner described is considered an additional innovation (bid'ah idafiyyah); this is due to likening something that is not sunnah to something that is sunnah. Those who perform this act must be advised and it must be clarified to them that this action was not transmitted from the Prophet (peace be upon him) nor from the righteous predecessors (Salaf al-Salih).