The "Toxic" singer left the Promises Treatment Center in Malibu, California, for her first supervised outing.
The 25-year-old filed for divorce from Federline in November and kept her wedding ring finger bare while dating, but has been wearing a band on her left hand over the last week.
Despite the couple's rocky two-year marriage, a source close to Federline tells Us Weekly magazine that once Spears recovers, "Kevin wants to get back together with her.
"He is her biggest supporter."
On Feb. 24, just three days after the star checked into rehab, Federline, Spears' mother Lynne, and the couple's young sons, Sean, 17 months, and Jayden, six months, dropped by the facility for a two-hour visit.
The source adds, "It was a very emotional and happy family moment."
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