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

 

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
Allaah.

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