Hypothetical-More Info
Two year old subdivision with monuments? I bet the neighbor who crossed the line to tie into an existing fence knew where the true line was located and probably knew that monuments were in place. More than likely knew all about adverse possession or at least thought he did. Were I the one who built the fence 1 foot off, I would remove it and sell the salvage. Let the other guy build the fence along the property line and make sure that is where it gets built. The other option would to create a document allowing the neighbor use the one foot without claim of ownership. That is not likely because there probably now is animosity between the two, one feeling that there was an attempt to take advantage of him and the other mad because he got caught.
More to this story than Hypothetical it seems. Unless you have sound reason to get involved, don't, if a friend or relative of the one who built the first fence, advice him to see a good land attorney and determine his rights and options to correct the problem, a simple letter from a good land attorney might put it all to rest, has happened that way for me.
jud
Hypothetical-More Info
Fence contractor got job because their price was lower by one side. Makes sense, why have two on (in the vicinity) the same line. But they need to have an agreement about maintanence and who owns what (including the common fence line).
Hypothetical-More Info
In my state the fence would need to be treated as the boundary for 20 years before it could be considered the established boundary and there are other requirements.
I say it's a good idea to make all parties aware of the location of the boundary markers. I've been surprised through the years at some of the calls from folks in new subdivisions that are unaware of their boundary markers. The 1 foot offset is quite common as we all know. UDOT does it. RR's do it and lots of private owners do it to make sure the fence is on their property. Had a client last year that was going to offset 10 feet so they could be on their land even on the other side of the fence. I told them to make sure the "real" corners are well marked. Always make new owners of adjacent land aware of where the corners are.
I remember, from the old RPLS.com board, someone posting about a municipality in PA, imposing the 2' law and 4' wide weed patches every where.
But maybe, as Junior suggests, it didn't last very long......