Although October 26 marked Keith Urban’s 39th birthday, the occasion was less than celebratory. While Urban spent the day in an undisclosed treatment center for alcohol abuse (he checked in October 19), his wife of four months, Nicole Kidman, 39, dined at Beverly Hills restaurant Matsuhisa, where she ordered sashimi alongside a group of her Australian relatives.

“It was a quiet dinner,” says an observer of their one-hour meal.

“Nicole seemed happy to be with her family.” And her friends: The night before, Kidman took her nephew for dinner at Naomi Watts’ L.A.-area home.

As for Urban, a source close to the singer says he may have HAD to go to rehab because of a condition in his prenup with Kidman, who shares two adopted children, Connor, 11, and Isabella, 13, with ex Tom Cruise, 44.

“Nicole’s lawyer put this clause in to demonstrate to Tom that Keith’s past problems wouldn’t affect the kids,” says the source. (Urban did a stint in rehab for cocaine addiction in 1998.)

No matter what, the singer has already won the support of the country-music community.

“He’s passionate, and it’s easy to kind of get off track when you feel so many things,” Dolly Parton tells Us. “He’ll be back soon.”