Imaam being corrected by a member of the congregation if he makes a mistake in the prayer
Question

                My father wanders if, during Salat Alfarthd, the Imam mistakes or forgets what comes next in the middle of soorah, can the ones behind him correct or help him out??
Praise be to Allah.This 
          is known as the issue of “correcting the imaam”.   
Correcting 
          the imaam is divided into two categories:  
That which is obligatory, which is correcting the 
          imaam concerning something which would invalidate the prayer if done 
          deliberately. If he makes a grammatical mistake which changes the meaning 
          of al-Faatihah, then he must be corrected, because this type of mistake 
          which changes the meaning of  al-Faatihah invalidates the prayer. 
          Similarly, if he omits a verse of al-Faatihah, he must be corrected, 
          because that invalidates the prayer. The kind of correction which is 
          mustahabb (recommended) is when the imaam makes a mistake which does 
          not invalidate the prayer altogether, but it does undermine the perfection 
          of the prayer. So if the imaam forgets to recite another soorah after 
          al-Faatihah, alerting him to this fact is sunnah.
The 
          evidence for that is the words of the Prophet 
          
          (peace and blessings of Allaah be upon him): “I am a human being like 
          you, I forget as you forget. If I forget, then remind me.” (narrated 
          by al-Bukhaari, al-Salaah, 401). So he commanded them to remind 
          him. On one occasion when he became hesitant in his recitation, he said 
          to Ubayy ibn Ka’b, “What stopped you?”, i.e., what stopped you from 
          correcting me? This indicates that correcting the imaam is something 
          that is essential.
See 
          al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, part 3, p. 346-347