"She could reveal what Ben said about Jen Garner in unguarded moments!" said a source.
Affleck, 46, was so out of it when they met, he never made Sexton, 22, sign a nondisclosure agreement, RadarOnline.com insiders revealed — leaving her free to talk about their hookups.
If Sexton were to pen a tell-all, publishing sources added, she could easily make up to $1 million.
And Affleck had better tread carefully, as he's enraged Sexton's mother, Suzette, who said, "Would you be OK with your daughter dating a man with 3 kids, 3rd time in rehab, and unstable as F?"
As Radar reported, Affleck was first spotted with his much-younger girlfriend in August, after he split with Saturday Night Live producer Lindsay Shookus.
The A-lister recently admitted he is "fighting" for himself after a 40-day stay in a treatment center for alcohol addiction. He remains in outpatient care.
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