Is what that person did — interrupting the prayer and going to perform ablution after his ablution was nullified in the second rak'ah of the Maghrib prayer, then praying with another man who arrived late — correct, or would tayammum have sufficed him, and what about the reward for congregational prayer, and is his prayer with that man considered a congregational prayer, and what if he had prayed alone?

Tayammum is not permissible except in specific cases that do not include attaining the virtue of congregational prayer. What you did, stopping the prayer when your ablution was invalidated and returning to the congregation, was correct. You should have joined the Imam to attain the virtue of congregational prayer, even if you did not catch a raka'ah. Your attaining the virtue of congregational prayer with another person is correct, and if you do not find anyone, your prayer is valid. Whoever says "Allahu Akbar" before the Imam's first tasleem has joined the congregation.