A source tells ET that Pitt “wanted to support” Aniston because the former couple have “been friendly” in the years following their split.
According to the source, Aniston put her ex-husband on her invite list weeks ago and Pitt was in town and happily chose to attend to help her celebrate the milestone birthday.
Though he was at the party, Pitt and Aniston didn’t have much interaction at the event. They were, however, seen briefly talking and exchanging a hug. The source adds that Pitt did not stay at the bash the entire night.
“He came to support her, spoke to other friends of his that were there and headed out,” the source notes.
Another source tells ET that Pitt was "having a great time" and spent most of the night hanging out with George Clooney and Orlando Bloom.
The father of six went to great lengths not to be seen leaving the party, and hotel staffers and security helped make it difficult for him to be photographed.
A source adds, "Jen and Brad have remained friends and have spoken with each other quite a few times since they both became single. They are friends and have supported each other through some tough times. They've come so far since their marriage and plan to stay friends for life. There are truly no hard feelings between them... They both realize that anytime they cross paths people talk about a reunion. He came on his own and quietly entered the party. They had heavy security, kept a ban on social media, and hoped for the best."
Pitt and Aniston were married from 2000 to 2005. Not long after their breakup, Pitt started dating Angelina Jolie and years later, they were married. The former couple share six children together and Jolie filed for divorce in 2016.
A source told ET last February that Pitt and Aniston do stay in touch after they both split from their spouses. Aniston and now ex Justin Theroux announced their separation in early 2018.
“They have each other’s cell phone number and communicate from time to time," the source said at the time. "This is nothing new. Jen and [Brad] have remained friendly."
As for just how friendly these exes are, a source told ET after Aniston called it quits with Theroux that the actress and Pitt were “absolutely not dating.”
In addition to the 55-year-old actor being at Aniston's birthday party, other celebs in attendance included Katy Perry, Ellen DeGeneres, Reese Witherspoon, Robert Downey Jr., Jason Bateman, and Pitt’s ex-fiancee, Gwyneth Paltrow.