You may have read it on the Curious Books site in Ed Sams' Lizzie Borden: Unlocked". I don't know the exact date when this was published but the bibliography refers to books published in the 1990s.
http://www.curiouschapbooks.com/cntnts.html
The Bridget confession story was originally published in Radin's book in 1961. It was reported in Lincoln 1n 1967 and Brown in 1991. The story has evolved since Radin's original version. This is how Radin reported it:
"The strange episode occurred during Bridget's serious illness in Anaconda when she was stricken with pneumonia and thought she was dying. When Bridget first emigrated from Ireland, she came to this country with a girl friend from her own home town. They separated soon after landing; Bridget settled in New England, and her friend went on to Montana. They corresponded irregularly over the years. The friend married and became Mrs. Minnie Green. Bridget never wrote to her friend about the Borden murders and Mrs. Green did not hear of the case at the time it happened.
When Bridget finally came to Montana and settled in Anaconda, she resumed her friendship with Mrs. Green, who was living in Butte, about twenty-seven miles distant. The friends visited each other but Bridget still did not mention the Borden murders to her childhood companion.
In 1942, when they were both about seventy-five years of age, Mrs. Green received an urgent telephone call from Bridget who said that she was dying, wanted to see her friend, and had a secret she wanted to confide to her before she passed away, Mrs. Green had to make arrangements to get to Anaconda and this took her a little time. When she arrived, Bridget had passed the crisis in her illness and she recovered shortly afterward.
Several days after visiting Bridget, Mrs. Green entered the Butte Public Library.
Miss O'Meara noticed an elderly woman staring at the stacks of books and in obvious need of assistance. The woman hesitantly asked if the library had any books on real murders, and when she learned there were quite a few, asked if anything had been written on the Lizzie Borden case. She told the librarian how Bridget had sent for her and related what occurred during her visit to Anaconda. While the principals were alive, Miss O'Meara never revealed the story; she did not even tell anybody that Bridget Sullivan was living in the area. Since both Bridget and Mrs. Green were dead, she consented in 1960 to tell me the details.
Mrs. Green told Miss O'Meara that Bridget had informed her old friend for the first time that she had been a witness in the Borden case. Bridget said that her testimony was favorable to Lizzie who, to show her gratitude, had given her money to visit her parents in Ireland, and added that the Borden lawyer had advised her to remain in Ireland and never return to the United States. Bridget said she bought a farm for her parents and stocked it with horses, cows, pigs, chickens and sheep. Later, Bridget said, she became restless, obtained a passport under another name and returned to this country, going to Anaconda. Mrs. Green said that Bridget told her she was fond of Lizzie and frequently took her part in family disputes. Bridget also said she had testified only to the truth at the trial.
Mrs. Green, who had known Bridget since they, were children, was frankly skeptical of her friend's story. She failed to find anything in it to account for Bridget's urgent demand that she rush to her bedside because she wanted to confide something to her before she died. Mrs. Green borrowed several books on the case from the library and, returned them several days afterward. When Miss O'Meara asked if she had learned anything, the older woman shook her head, looked puzzled, and left. As far as Miss O'Meara knows, she never came to the library again. Bridget recovered and moved to Butte. Whether the two old friends saw each other after this episode is also unknown.
Believers in Lizzie's guilt can seize upon this incident to bolster their claim that Bridget withheld information and was paid off by Lizzie. It also would help substantiate the stories about her return to Ireland with a lot of money. Bridget's statement to Mrs. Green is the only evidence that she may have gone back for a time. Bridget still had relatives living in Fall River after the trial and could have heard from them what was being said about her.
There are many reasons to be skeptical of the story Bridget told Mrs. Green. As Mrs. Green realized, Bridget had really said nothing which could explain why she had asked an elderly friend to hurry to her bedside. Her claim that she interjected herself into family disputes to side with Lizzie is unbelievable. So is her insistence that she told only the truth at the trial. And despite what she told Mrs. Green, her trial testimony, as has been shown, was anything but favorable to Lizzie. There are also serious discrepancies in her testimony concerning that vital half hour of time unaccounted for on the morning of the murders. One can only speculate as to what Bridget might have told Mrs. Green had she still thought she was dying when her friend finally arrived."
I know of no evidence that Bridget was paid off. I agree with Radin - there are many reasons to be skeptical of this story,