Is it obligatory for someone who interrupted their prayer due to an elderly man's distraction, and then re-offered it individually, to inform the congregants who prayed with them that they must repeat their prayer?

Intentionally interrupting an obligatory prayer without a legitimate Shar'i (Islamic legal) excuse is forbidden by scholarly consensus, based on the Almighty's saying: "And do not invalidate your deeds" (Qur'an 47:33). The continuation of intention is a condition for the validity of prayer, and whoever intends to interrupt it renders it void. Thus, whoever interrupts their intention and completes the prayer without an intention has committed a forbidden act and their prayer is invalid. They must repent and restart the prayer.

As for the obligation of informing the worshippers after the prayer has ended, there is a difference of opinion on this matter. However, the most preponderant view—and Allah knows best—is that the prayer of the congregants is not invalidated because they are excused due to their ignorance, and they are not burdened with inquiring about the imam's purity. Al-Uthaymeen and Al-Sa'di stated the validity of the congregants' prayer in this situation because they performed what they were commanded to do, and they are not bound by a ruling that they were not commanded to observe. Also, the default state of their action is validity until its invalidity is proven by a Shar'i evidence. Therefore, you are not required to inform the worshippers because their prayer is valid according to the preponderant view.