The actress's husband of 12 years, Michael Douglas, is fully there for her as she enters treatment once again for her bipolar II disorder, friends say.
"He couldn't be more supportive, just like she is of him. They're solid," a friend tells PEOPLE.
The friend adds: "They're really, really devoted to each other."
Douglas, 68, and Zeta-Jones, 43, stepped out together on April 22 in New York for the Film Society of Lincoln Center's 40th Annual Chaplin Award Gala, honoring Barbra Streisand. They walked the red carpet together, and attended the tribute and a dinner afterwards. Fellow attendees said nothing seemed amiss.
"Catherine was on his [Michael's] arm much of the time," said one. "She seemed really relaxed and happy. I saw her with a cocktail and she was socializing and enjoying the evening … Nothing seemed out of the ordinary, and they both seemed to be having a good time."
The couple have weathered difficult times before. In 2011, friends said their strong relationship helped them both through separate health problems – as he recovered from throat cancer and she first sought treatment for bipolar II.
At the time, Douglas told PEOPLE of his wife: "It takes a lot of courage to seek help, and I am very proud of Catherine for doing something positive about her situation. It's onwards and upwards for us both."
Reporting by SHARON COTLIAR