Update:
My sister talked with an attorney friend of hers and was told that the fence had to be in place for 25 years without dispute. Before we could write a certified letter and/our talk to the neighbor again, he already had workers digging up the old fence posts and moving the fence back to the surveyed property line. We don't know if he talked to an attorney who told him the same thing or what. In the end, it all worked out well.
My sister talked with an attorney friend of hers and was told that the fence had to be in place for 25 years without dispute. Before we could write a certified letter and/our talk to the neighbor again, he already had workers digging up the old fence posts and moving the fence back to the surveyed property line. We don't know if he talked to an attorney who told him the same thing or what. In the end, it all worked out well.