Flew himself answered the critics accusation about the work being varghese's work. A discussion of that can be found here: http://www.bethinking.org/science-christianity/intermediate/flew-speaks-out-professor-antony-flew-reviews-the-god-delusion.htm
Reading his review of the god delusion doesn't strike me as the work of a senile old man.
according to this link: http://www.christianpost.com/news/renowned-atheist-turned-deist-antony-flew-dies-at-87-44761/
The 2007 book, There is a God: How the World's Most Notorious Atheist Changed His Mind, describes Flew's conversion from atheism to deism and is thought by some to be the work of Varghese that Flew merely signed off on.
New York Times Magazine writer Mark Oppenheimer expressed his doubts over Flew's mental capacities after meeting up with him in England, suggesting that the once great philosopher had become a "blissfully unaware" old man "just following the evidence as it has been explained to him."
"Depending on whom you ask, Antony Flew is either a true convert whose lifelong intellectual searchings finally brought him to God or a senescent scholar possibly being exploited by his associates," he wrote.
Flew, however, released a statement rebutting the circulating allegations, saying that he would not have a book issued in his name that he does not 100 percent agree with.
“I needed someone to do the actual writing because I’m 84 and that was Roy Varghese’s role,” Flew stated. “The idea that someone manipulated me because I’m old is exactly wrong. I may be old but it is hard to manipulate me. That is my book and it represents my thinking.”
The book, notably, went on to win the 2008 Christianity Today Book Award in Apologetics and Evangelism, with the judges hailing it for making the philosophical search for God both “accessible and exciting.”
Read more at http://www.christianpost.com/news/renowned-atheist-turned-deist-antony-flew-dies-at-87-44761/#Zjtu1bkomWpDXfly.99