Mel Gibson and Oksana Grigorieva Split

Mel Gibson and Oksana Grigorieva, his Russian musician-singer girlfriend and mother of their 5-month-old daughter, have split after over a year of dating, a source tells PEOPLE.

Oksana Grigorieva and Mel Gibson

"They just drifted apart," says a close friend of the couple of the amicable split. "They’re both working hard on their careers and trying to raise a sweet baby together. They're still friends and they'll both raise Lucia together."

Gibson is in Mexico filming How I Spent My Summer Vacation, and Grigorieva is working on her music. Next week, she has plans to perform a charity concert in Russia to benefit "Grant Life," a foundation to help children with cancer and other life-threatening illnesses.

In addition to Lucia, who was born on October 30, 2009, Gibson, 54, has seven children (six sons and a daughter) with his estranged wife Robyn, while Grigorieva has a son, Alexander, with her former boyfriend, actor Timothy Dalton.

In April of last year, the actor's wife of 30 years filed for divorce. Later that month, he and Grigorieva made their first public appearance together as a couple.

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