"She knows who David is and what he's like," a source close to the couple tells PEOPLE. "She married him. All that he said, and how he said it, that's David."
"Does she wish she could edit what comes out of his mouth? Sure," the source continues. "But nothing surprises her at this point. What's amazing about Courteney is that she doesn't get caught up in all the ridiculousness."
On Tuesday, after news of their separation broke, Arquette, 39, chatted with Stern about his 11 year marriage to Cox, 46, denied rumors that he cheated on her and said that she stopped having sex with him months ago.
Why did he choose to air all that dirty laundry on the radio?
"He felt really attacked and his first instinct was to defend himself," the source says. "He was totally bombarded with a lot of negative attention, most of which was inaccurate."
And Arquette, the source says, isn't taking his separation lightly. "It's incredibly difficult to go through [it] especially with all the speculation about what happened," the source says. "It's David's instinct and way of reacting to tell the truth."
Says the source: "He wanted to make it clear that he had been faithful for 11 years and has only been with another woman [Jasmine Waltz] after he and Courteney were [separated]."
As for the split, which was surprising to many, close friends saw problems brewing.
"This was a long time coming," the source says. "They haven’t been happy for a while and have been trying to make it work but they just can't do it anymore."
Reporting by JENNIFER GARCIA and JULIE JORDAN