Jennifer Aniston and Justin Theroux Separately Attend Jimmy Kimmel's Son's Birthday Party

Looks like Jennifer Aniston and Justin Theroux have both been able to maintain their close friendship with Jimmy Kimmel after announcing their split in February.

Kimmel celebrated his son, Billy's, first birthday with a party at his Hollywood Hills home on Saturday, and an eyewitness tells ET that both Aniston and Theroux attended the birthday bash, though it is not known if the exes crossed paths. According to the eyewitness, Aniston arrived at the party at 12:15 p.m. and left at around 2 p.m. -- still wearing a hand brace on her left wrist -- while Theroux spent about 40 minutes at the party.

Aniston was snapped arriving at Kimmel's home on Saturday, dressed casually in a black tank top, cropped jeans and black sandals. 

Meanwhile, on Sunday, Theroux Instagrammed adorable photos of himself with the birthday boy, praising both Kimmel and Kimmel's wife, Molly McNearney, for being vocal about health care access following Billy's multiple heart surgeries to correct a heart defect that he was born with.

"Happy ONE year birthday Billy!" Theroux wrote. "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. ❤️ Thank you @CedarsSinai & @ChildrensLA #registertovote."

Kimmel also Instagrammed a picture of Billy from his birthday party, writing, "A yr ago today, God gave us this tough little boy. Today, we thank the nurses & doctors at @CedarsSinai & @ChildrensLA who saved his life & those who shared thoughts & prayers. We cannot stay quiet. Please wish Billy happy birthday by REGISTERING TO VOTE! vote.gov."

Kimmel and McNearney have been notably close with both 49-year-old Aniston and 46-year-old Theroux over the years, with the late-night host even officiating their 2015 wedding and hosting Theroux's "G-rated" bachelor party. In March, Aniston made a rare post-split appearance at McNearney's 40th birthday party, where she posed for a photo with McNearney, jewelry designer Jen Meyer (Tobey Maguire's ex-wife), Amanda Anka (Jason Bateman's wife) and her manager, Aleen Keshishian.

Last month, a source told ET that Theroux has been hitting the gym hard following the breakup, and spending time with close pals as well.

"He's making a point to get out every day and to see friends," the source said. "He's been dining with friends regularly ... [He's] trying to move on, [and] he's dealing with this breakup in a really healthy way."

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