potential for blackmail and exposure is real, and also just mortifying embarrassment

This would go both ways though... If she has nudes of him, she could literally just blackmail him back.

Sorry if I seem uptight, I’m all for slutty kinks, but not with non consenting victims. I find his behaviour deeply concerning.

She is an adult fully and consciously aware that she is messaging someone on the internet that she knows nothing about and could be literally anybody. Just because OP managed to deduct the person's identity is not a crime in itself, nor should it be a problem.

You alone are accountable for YOUR ACTIONS. You. are. an. adult. I am not a fan of this infantilizing of adult women.

If you don't want the risks that come with sexting anonymously, then don't sext anonymously. You aren't being deceived, you can't possibly expect me to believe that you didn't know that sexting random strangers online meant you could possibly be sexting anyone. That is ridiculous. Even more so when you don't even bother to fucking validate who the other person is, or worse... bother to validate they are even an adult in the first place.

If we are talking about deviant behavior.... then frankly, OP's room-mate knows nothing about her partner; she could technically be sexting with a minor for all she knows. If she is frequently sexting other internet strangers, how many minors masquerading as adults has she sexted? That in itself, is criminal.

Now, maybe if OP was like convincing her to do 'x' action where she believes she is alone and hidden, and then he goes and peeps on her, that in itself would be actually criminal. But frankly, he has not broken any laws.