If they surveyed from the legal description (legal description takes precedence over a drawn line) of the land based on the deed, that is a good start. Where the neighbor may have some rights to it is if they have been paying taxes on that land for a number of years. Say they own 100 acres but have been paying taxes on the 105 acres for 50 years, whereas you own 105 acres and have been paying taxes on 100 acres. They could claim they were the maintainers of the land and have been paying taxes on it for a long time. A lawyer that deals with land ownership will know more on this, but I would try to check what your father has been paying taxes on vs. what he owns. He may have been paying taxes on that land even though he didn't know it was his. he could have been paying taxes on "100" acres, even though he only had access to 95. In that case, you would have very solid ground to stand on.
One way to possibly work it out with them is to grant a temporary easement allowing them to continue to use the land for a specific time period(say, the farmer who rents the land has it for another 5 years before his contract is up). At that point when they go to renew a rental contract for farmuse, the easement will expire and then they will no longer have access to said land.
If you don't mind telling me the county it is in, I can point you towards their online parcel system if they have one.