After A1 handed the ball off to A2, A3 (tight end)pulled down the line and pushed A2 into the line of scrimmage. This happened during a game that a friend of mine was the coach. He discussed this with the officials, the coach said it was assisting the runner. The white hat said that this was a legal play. Is this legal or illegal?

HTBT, but as written, illegal.

Things to consider - was he pushing anyone else as well? Was it intentional, did he just fall into him?

We had this situation a couple of years ago. The white hat threw the flag and we had a spot foul since the end of the run was beyond the spot where it first happened. He even got the signal right. I was impressed. Naturally the A coach was not happy. I think this happens more often than we think it does.