The ruling on one who intentionally or forgetfully leaves off standing in a rak'ah

Question:
The questioner says: I was praying while preoccupied, and there was another man beside me. The imam raised his head from prostration, and then we raised our heads, while the man was still in prostration. When the imam bowed, the man rose from his prostration and stood, then immediately bowed. What is the ruling on this rak'ah? And what is the ruling on his prayer?

Answer:
If he did not notice, he is excused, and what he did is valid. However, if he did it intentionally, he missed the standing, and this rak'ah is invalid; because he intentionally left out one of its pillars. Standing in the obligatory prayer, even for the length of Surat al-Fatihah, is one of its pillars. So if he intentionally left it out and remained in prostration until the imam bowed, then he stood up and bowed with him, this rak'ah is invalid; it becomes a desire.
Because he left out a pillar of it. As for if he did not intend it, but sleep took him, or... or something like ignorance or fainting overtook him, then he is excused. And if he bowed with the imam, the standing is dropped from him due to the excuse.