A hidden camera?
You don't need to get her caught, you can just as well refrain her from doing it.
Make up a fake camera and make sure she sees it or at least sees you installing it to look over the yard.
When the camera is outside, it doesn't have to look exactly like a camera, because outside cameras are put into a case which covers the camera from rain and such.
Put a glass cover in front and perhaps a lens from binoculars inside the case, just to make it look like a camera lens.
The cameras first function is to work as a deterrant and the secondary function is to work as an aid in identifying a subject.
If the deterrant works, you'll save money.
If doesn't, then you can consider a real camera.
Either ways, you probably wouldn't get much out of it even if you'd get her caught in the act.