What is the ruling on the prayer of someone who caught the second rak'ah and then stood to make up what he missed before the Imam's Tasleem, out of ignorance?

Scholars have differed regarding one who intended to separate from his Imam before the Taslim (definition) and completed his prayer individually. Some consider it valid absolutely, while others consider it invalid absolutely. Some differentiated between separating with an excuse (in which case it is valid) and without an excuse (in which case it is invalid). The preponderant opinion is that it is valid, especially with ignorance. However, if he stood up to make up what he missed while still being a follower of the Imam, his prayer is invalid due to his egregious opposition to his Imam, and he must repeat it.