"He phoned her the day before it came out and he hasn't been in touch since," Mary, 72, told Grazia. "She can't get hold of him. She's absolutely devastated. This is so out of character for the Liam I know."
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Beady Eye frontman Liam has denied fathering the seven-month-old daughter of New York-based journalist Liza Ghorbani, who is suing him for £2 million in child support.
But the scandal has left Nicole feeling betrayed, as she continues her holiday in Florida with her three sisters, Natalie, Lori and Lee, her son Gene and Liam's 13-year-old child Lennon from his marriage to Patsy Kensit.
The former All Saints singer, who has been pictured without her wedding ring, is being supported by her family, who have all rallied around her. "Her sisters are supportive, but she is in a terrible state," Mary said. "It's like he's lost his mind… it's been a terrible shock. It feels like he's been living a double life."
She went on to say that while she "won't say never" to the idea of her daughter reconciling with Liam, "at the moment, Nicole has no interest in having him back."
The New York Post broke the news that Liam, 40, had reportedly fathered a love child with Liza. The musician has instructed his lawyers to pursue legal action against the newspaper for naming him in the article.
Only a few days after the scandal broke, Liam was photographed laughing and joking beside a pool in Ibiza with his personal assistant Debbie Gwyther. She was reportedly fired by Liam's management the following day for being "too close" to the star, but Mary also said that Nicole believes the pair are now together.
"Nicole knows they are together. She has had it confirmed," she said. "We believe he's just dumped Nicole and gone… I believe he doesn't want to save the marriage."
Nicole and Liam have been married since 2008 and have a 12-year-old son, Gene.