It is believed she ducked out of her birthday bash at around 1am in the morning.
"Pitt made a casual entrance to the hotel where Aniston's party was being held." the medium reported.
Meanwhile, Gwyn posted a similar series of images, captioning it: "Happy birthday (last night and tomorrow) to #jenniferaniston our ray of sunshine, example of perpetual goodness". The "We're the Millers" star was reportedly hesitant to invite Pitt, but wanted to celebrate her milestone birthday with those who were important in her life, according to People. "She had some family there and seemed very happy to be with so many good friends".
Other guests included Gwyneth Paltrow (another Pitt ex), John Mayer (another Aniston ex), Katy Perry (Mayer's ex) & Orlando Bloom, Courteney Cox, David Arquette (Courtney's ex), Lisa Kudrow, Barbra Streisand, George & Amal Clooney, Reese Witherspoon, Demi Moore, Laura Dern, Kate Hudson, Keith Richards, Ellen DeGeneres, Robert Downey Jr. and many more. "We love you so much and you make 50 LOOK".
"Basically, every single person who Jen loves attended".
Although Aniston celebrated on Saturday, her actual birthday is on Monday.
The medium said there were no reports of Pitt and Aniston mingling throughout the evening, though.More news: Ole Gunnar Solskjaer on the right track
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The A-list actress is also mother to 15-year-old son Ryder with rocker Chris Robinson, and seven-year-old son Bingham with her former fiance, Muse's Matt Bellamy.
In a 2005 interview with Vanity Fair, Aniston said she was "shocked" by the breakup and was trying to "pick up the pieces in the midst of this media circus".
News the former couple - who tied the knot in 2000 and spilt five years later - are friends but that's it.
"I don't have time to get her a present".