Affleck advised J.Lo on her crumbling marriage

Ben will always be in Jen's corner.

Although Jennifer Lopez and Ben Affleck famously called off their engagement and 18-month romance in January 2004, the actor was happy to help as his ex's 7-year marriage to Marc Anthony was falling apart.

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Guadalupe Lopez, the singer-actress's mother (and plus-one at the July 9 BAFTA bash with the royals in Hollywood), sought Affleck's advice on July 12, multiple sources confirm in the new Us Weekly, out Wednesday. "Guadalupe reached out to Ben over email. She wanted advice for Jennifer," reveals a source.

"She always liked and trusted him. [Ben] replied back on email, wished her well and offered what he could," the source says of Affleck, 38, now happily married to Jennifer Garner with two daughters. (Lopez, now 41, wed Latin crooner Anthony, 42, just months after she and Affleck split.)

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Three days after that secret email exchange with Affleck, Lopez and Anthony announced the end of their union in a joint statement. "It was a very bad marriage, and Jennifer got the courage to end things," an insider says, adding, "[Marc] was a terrible husband."

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At issue: Anthony's raging jealousy over Lopez's career, his compulsion to control everything from her business endeavors and even her famous wardrobe, and, according to multiple sources, at least one extra-marital affair in 2009.

And despite her ultra-glamourous, ultra-confident persona in the public eye, Lopez "actually has low self-esteem," one pal explains. "And Marc really played into that insecurity. That's why she put up with his abuse for such a long time."

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For much more on the shocking split -- what's at stake, their money squabbles, who will get 3-year-old twins Max and Emme and whether she's ready to date again -- pick up the new Us Weekly.