Is it permissible for someone who is not praying to correct the Imam if he makes a mistake in recitation or forgets something in his prayer?

It is permissible for someone who is not praying to correct the Imam or alert him to his forgetfulness. If a group whose information provides necessary knowledge alerts him, he must heed their words. Similarly, if two just individuals alert him, he must heed them. It is obligatory for the congregants to correct the Imam if he makes a mistake in reciting Al-Fatiha, or is unable to recite it, or fails to perform a pillar.