To get the Dog ending you have to get all 3 normal endings or just the Rebirth ending in previous playthroughs. Since the Rebirth ending is only available after the first NG, you need to finish the game at least three times to get the Dog ending. So if his friend got the Rebirth ending previously and he was using his NG+ instead of a new file he could have gotten it, but his friend would have needed to beat the game multiple times.