Dropping by the Sunset Tower Hotel in Los Angeles on Saturday night, the 55-year-old made a casual appearance at the Dumplin' star's party.
"Basically, every single person who Jen loves attended", the source said.
An insider told People: 'They hugged and chatted for a bit, but Jen was busy making sure all of her other guests had an fantastic time'.
And proving she stays amicable with more than one former flame Jen's ex-boyfriend John Mayer was also on hand to wish her well.
"I'm just going to go. eat her food and drink her wine and say 'Happy Birthday!'"More news: Ariana Grande wins first Grammy
"They hugged and chatted for a bit, but Jen was busy making sure all of her other guests had an awesome time", the source added.
PEOPLE previously confirmed that Pitt was spotted on Saturday at the celebration, which was also attended by Pitt's ex Gwyneth Paltrow as well as Aniston's Friends costars Lisa Kudrow and Courteney Cox.
Among the big names were Ellen DeGeneres, George and Amal Clooney, Orlando Bloom and Katy Perry, along with Reese Witherspoon, who partied until she literally fell down.
"We love you so much and you make 50 LOOK [fire emoji]", she wrote in the caption.
She certainly wasn't alone when she left last night's party - the star was surrounded by pals. "Happy BDay JA! We all love you sooo!"More news: Former Raptors star helps welcome Marc Gasol to Toronto
"Our ray of sunshine, example of perpetual goodness".
Would you believe us if we told you February 7, 2019 marks Jennifer Aniston's 50th birthday?
Then Pitt met Angelina Jolie on the set of Mr & Mrs Smith in 2003, ditched Aniston, had six kids and decamped to a French chateau with the actor-turned-humanitarian.
Speaking to The New York Times, Theroux revealed that it was "the most gentle separation, in that there was no animosity".
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".More news: Rajini dances at daughter's sangeet, CM attends wedding
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