Yeah, we learned that the hard way last summer. Get a proper survey done before anyone even picks up a post hole digger, it's worth the couple hundred bucks. We used pink spray paint to mark the exact line on the ground so there was zero arguing when we started.
Pink paint is smart for the initial dig, but Mary should also check if her county has online parcel maps. We drove metal stakes with concrete at the corners after our survey, way better than just paint for long term. Neighbors can't argue with a physical marker in the ground.