The Peppermint star, 46, appeared relaxed while wearing a black T-shirt, blue jeans, white tennis shoes and black sunglasses with her hair in a messy bun on Thursday.
Just the day before, Garner was photographed driving Affleck, also 46, to a treatment facility in Malibu.
His decision to re-enter rehab came two days after he was seen receiving a box of alcohol at his Brentwood home with what appeared to be Johnnie Walker Blue Label scotch whisky in a blue case and some other unidentifiable bottles.
A source told PEOPLE this week the actress is most concerned about the three children she and Affleck share — Samuel, 6, Seraphina, 9, and Violet, 12.
“Her number one concern is the kids,” the source says, noting Garner has been hoping to finalize their divorce soon, but “struggles” with questions like, “Is Ben healthy enough to have custody? Can she rely on him to co-parent?”
The insider added Affleck “knew he needed help and was vocal about it.”
Affleck’s return to treatment comes after he was spotted out with 22-year-old Playboy model Shauna Sexton in recent days.
Garner’s focus remains on their three children and Affleck’s health, with the source saying, “What she ultimately cares about is his sobriety. Her concern right now is not who he is dating.”
RELATED VIDEO: Ben Affleck Enters Rehab as ‘Visibly Upset’ Jennifer Garner Drives Him to Treatment
As their children head back to school, a second source tells PEOPLE the situation has been “very stressful for her.”
“And also devastating for her to have to stage an intervention in the middle of the afternoon, pretty much in public,” the insider adds. “But this was a crisis situation.“
The actor previously spent time in rehab in 2001 and 2017. “He knew he needed help and was vocal about it,” a source told PEOPLE.
Affleck wrote on Facebook in 2017 after a previous stay in rehab: “I want my kids to know there is no shame in getting help when you need it, and to be a source of strength for anyone out there who needs help but is afraid to take the first step.”