I havent' heard that the writer did not research the film.
I did see this:
"While Gillespie -- who acknowledged that he has not seen "The Reagans" and has formed his opinion of it based solely on news reports -- had a number of complaints, he said he was most concerned about a comment attributed to Reagan in one episode. There is no evidence that the president told his wife during a conversation about AIDS patients, "They that live in sin shall die in sin," Gillespie said.
The author of the screenplay, Elizabeth Egloff, has acknowledged that there is no evidence Reagan ever uttered those words, but she told the New York Times that "we know he ducked the issue over and over again, and we know she was the one who got him to deal with that."http://edition.cnn.com/2003/ALLPOLITICS/10/31/reagans/
edited to add: Not brilliant on her part, I will admit. I also heard Chris Mathews on MSNBC take issue with this statement too. He felt it was very unlikely that RR said it.