It really is, though. She thought she could go through his things and he thought he didn't have to tell her what he drew.
He should have been upfront about it and she shouldn't have looked without his permission.
I agree with your point that she shouldn't go through his things...
However context is important here... they have a dynamic where she normally does that...
Like she's been going through his things since issue 1 bro. that's their normal. So that isn't her doing something wrong against him, because that is the dynamic he's allowed and fostered. (implicit permission basically, like you don't ask your girlfriend to kiss her every single time... after the 10th it's the norm).
And it isn't like she does it maliciously, she's legit curious about him, and what he does.
You know what isn't their normal? paisen lying to her.
That's the mistake, that's the issue, it wasn't that he dude a nude painting (like he wrongly thinks) it's that he lied to her.