The Friends star, 45, spent a couple hours at the Hollywood party mingling with guests and sending her best wishes to the birthday boy on Saturday. The Leftovers actor, 46, was spotted leaving Kimmel’s home shortly before his ex arrived.
“Jen is doing well. She is keeping busy and moving on with her life,” a source tells PEOPLE.
Theroux dedicated a special birthday tribute to the child on Instagram over the weekend, posting four adorable photos of himself with Billy. Alongside the images, Theroux wrote a message about his health battles over the past year.
“Happy ONE year birthday Billy! 🎂,” he captioned the snaps. “A boy who in 365 days has done more for American national health care than we could all wish to do in our lifetimes. He doesn’t even know yet how courageous he is. An honor to know him and his incredible and equally courageous parents. ❤”
Theroux finished the message by thanking Cedars-Sinai Hospital and Children’s Hospital Los Angeles and reminding fans “#registertovote.”
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Kimmel and his wife revealed that their son underwent two heart surgeries after he was born with a congenital heart condition, tetralogy of Fallot with pulmonary atresia. Last May, the two-time Oscars host announced during a tearful address that Billy had open heart surgery at three days old as a result of his son’s “pulmonary valve [being] completely blocked and a hole in the wall between the left and right sides of his heart.”
Billy had a second surgery at the Children’s Hospital Los Angeles in December, seven months after his birth. (Kimmel and McNearney also share 3-year-old daughter, Jane. He also has two older children – Kevin and Katie – from a previous relationship.)
The comedian used his platform to become an advocate for universal health care coverage, as well as other political issues, including gun violence prevention.
Meanwhile, a source previously told PEOPLE that since splitting from Theroux, Aniston’s “life now is not very different” than it was before. “Justin has moved on with his life in N.Y.C.,” while “Jen still enjoys hers in L.A.,” the source continued, adding that Aniston “loves socializing with friends.”
Aniston and Theroux announced that they had split after two and a half years of marriage and seven years as a couple on Feb. 15.