To the OP...
Again, this was way back when I did taxes and read articles about taxes...
There was some group that had a contest with very complicated issues... professionals would submit what they thought the return should look like... IIRC, there were hundreds of submissions, but only ONE was prepared correctly.... to be fair, they were throwing in things that even most tax accountants do not every see....
Do you know IF he actually had the return 'done' on the 13th
Sometimes they just do a quick estimate and if they miss some limitation of a loss or something else that estimate could be way off.... also, they might not have looked at AMT.... and you could very well be paying a big amount of AMT... so your regular tax could be low, but throw in AMT and you get hit hard...