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He most definately has her nudes, and he most definately will never leak them.
The worst thing he could probably get away with doing is leak personal information that would result in her dox, or (less likely) private chatlogs that would upset or embarass her.
It's a moot point anyway; the "blackmail" hypothesis is just a cope to desperately cling to the idea that she's being somehow coerced against her will, as opposed to fully and willingly participating in this unmitigated disaster of a relationship.