Even if true, this particular argument would make no difference to the ongoing creation vs evolution debate. If one person changes his mind, does that make the system untrue? Would you argue that because one Christian recanted, Christianity is false?
In any case, this particular idea is not true.
Note: This is only here until I have researched this for myself.)
A few more details on the spread of the story and its subsequent rebuttal, taken from the book "The Survival of Charles Darwin: a Biography of a Man and an Idea" by Ronald W. Clark, published by Weidenfeld & Nicholson, 1985 (p. 199)
Shortly after his death, Lady Hope addressed a gathering of young men and women at the educational establishment founded by the evangelist Dwight Lyman Moody at Northfield, Massachusetts. She had, she maintained, visited Darwin on his deathbed. He had been reading the Epistle to the Hebrews, had asked for the local Sunday school to sing in a summerhouse on the grounds, and had confessed: "How I wish I had not expressed my theory of evolution as I have done." He went on, she said, to say that he would like her to gather a congregation since he "would like to speak to them of Christ Jesus and His salvation, being in a state where he was eagerly savouring the heavenly anticipation of bliss."
With Moody's encouragement, Lady Hope's story was printed in the Boston "Watchman Examiner". The story spread, and the claims were republished as late as October 1955 in the "Reformation Review" and in the "Monthly Record of the Free Church of Scotland" in February 1957. These attempts to fudge Darwin's story had already been exposed for what they were, first by his daughter Henrietta after they had been revived in 1922. "I was present at his deathbed," she wrote in the "Christian" for February 23, 1922. "Lady Hope was not present during his last illness, or any illness. I believe he never even saw her, but in any case she had no influence over him in any department of thought or belief. He never recanted any of his scientific views, either then or earlier. We think the story of his conversion was fabricated in the U.S.A. . . . The whole story has no foundation whatever." (Ellipsis is in the book)
Clark's source for Lady Hope's supposed quotations of Darwin is given as "Down, the Home of the Darwins: The Story of a House and the People Who Lived There" by Sir Hedley Atkins KBE, published by Phillimore for the Royal College of Surgeons of England, 1974.
Henrietta's rebuttal is referenced more fully as: Mrs R B Litchfield, "Charles Darwin's Death-Bed: Story of Conversion Denied," "The Christian", February 23, 1922, p. 12.