I spent almost ten of the last eleven years as a field surveyor. Boundary is complicated.
We don't have any information about what kind of survey the property owner had done. We don't know the legal description of either property. It's possible the neighbor's survey may show the fence in the correct location. It's possible that the fenceline begins on the correct line and diverges over distance. Is the disputed swath 1/8, 1/4, 1/2, or 1 mile?
There is not enough information here to make those determinations.
I am well versed in the line work shown on apps like OnX.
So no, it's not as simple as going out and pounding some posts.
Every guy who knows a guy that disputed a property line thinks they're an expert in legal boundaries. The more you actually know about it, the more you realize the nuances.