I have a question about boundaries that I can’t find Info on. I’m curious, if someone has a boundary, a limit for acceptance of something but it conflicts with someone else’s boundary how is that worked out, or is it?

For example say someone doesn’t want a partner who looks at porn, it just doesn’t work for her. Then she unintentionally find out her SO uses it a lot. He knows it doesn’t work for her in a relationship and so because of that fact he’s always hidden it and lied about what he’s doing. He tells her he has stopped but obviously she has no way to verify that or to know if her boundary is still being broken or not or if he’s still hiding it and lieing. She can’t know whether the relationship is what she wants or not.
So his boundary is that he wants total privacy, so if she looks to see if her boundaries are being violated she is then violating his boundaries. Is this what would be an irreconcilable difference or what can be done in a situation like that.?
It’s usually easy to know if your boundaries are being violated becuase it’s visible. But what about those such as cheating, going places or with someone or doing something that can be hidden from you? Or is the boundary really that of being dealt with honestly so the minute someone does that they are breaking the boundary because without it then there is no way of knowing if the other things are happening?
I’m new to this boundary thing and am confused!