The assistant would have been about 15 yards out when the first shot went in and allowing for a 'decent' turn of pace that would be down to 8 or 9 yards where he would not have had any angle to see. As the gk messes about with the ball and it appears to go back over the line the post would now be in the way as the assistant still would not have made the goaline. At no point and in my opinion through no fault of his own, he was in no position to make a conclusive decision. Sat in the stands you would have had a better view