Ruling on the follower sitting while the imam is standing due to forgetfulness

Question:
A man is praying with the imam and a group of people. The imam prayed two rak'ahs and forgot the middle sitting, so he stood up. The one asking, along with a group of people, was in a spacious mosque, so they sat for the tashahhud. When they heard the takbir, they stood up. When they stood up, they found that the imam had bowed and raised his head from the bowing?

Answer:
Do they bow and catch up with his bowing? Did they bow?
The student: No, they did not bow; they followed the imam in this state.
The Shaykh: They repeat the prayer if a long time has passed. If they remembered immediately, they perform a complete rak'ah.
Q: Has this been three or four days?
The Shaykh: They repeat it; because they left out a rak'ah.