Something momentous happened to him during that winter that he felt caused him to want to change the course of his life in a spiritual way.
Washington did not have any such change in the course of his life. He was a steady Anglican and later Episcopalian, and there is no evidence whatsoever that either his religious philosophy or public position changed at any time throughout his life.
If he saw this "vision" it left so little effect on him that he changed neither the routine nor practice of his worship; not even a single iota. He was so little moved by it that there is no record that he even so much as hinted at it to another living soul.
The story is a hoax, and its author was a liar.