I would go after the realtor. If they sold you a house with X sqft, and you actually get Y sqft, I would say the realtor is liable.

The realtor will blame the title insurance firm and will be out of the picture. The title insurance firm will claim they did nothing wrong, and that it is the landowner's actions (or inactions) that have caused the landowner's losses.

Which in this case, it is; the owner sounds particularly upset, but as he tells it the fence was put up shortly before he bought the property. He has sat there for decades without ever having measured it up. Adverse possession takes a long time to come good. All advice about the purchase of property is always to get it surveyed.

And the owner had a few remedies. The easiest is to quickly grant a revocable right to put a fence there. This makes the possession not adverse (there is no trespass). That makes it temporary.

And, the landowner might have passed away.

I understand the reason why someone might not trust the most local government (municipal, state, federal governments as appropriate) to manage the registry of landownership complete with dates, dimensions, notables features, and owners, but I find it really weird that problem of knowing exactly who owns what is effectively an orphan in the last resort.