What is the ruling on the prayer of one who caught the Imam in the second rak'ah of the 'Isha prayer, then the Imam forgot to recite Al-Fatihah in that rak'ah and repeated it, so the follower completed four rak'ahs with the Imam, then added a fifth rak'ah after the Imam's taslim?

The more preponderant opinion is that Al-Fatiha is a pillar of prayer, and prayer is not valid without it. Whoever forgets Al-Fatiha or part of it, and then realizes it, must go back to recite it, unless he has already reached its position in the following rak'ah. In that case, he should complete his prayer and perform an additional rak'ah because the rak'ah in which Al-Fatiha or part of it was omitted is invalidated. If the Imam performs an additional rak'ah to replace the invalid rak'ah, you, as a follower, must follow him. Then, after his tasleem, you perform the rak'ah you missed, and your prayer will be valid.