If her ownership of the land on which the fence is placed has not been questioned for over 10 years, the land is hers by default, at least in Utah or in Idaho. I would suggest that you speak to a lawyer & lay out the facts above, & pay to have a letter sent to the neighbor.  He bought the land, knowing where the fence was, so he hasn't much ground on which to stand.  I suspect just the letter from a lawyer will solve the problem.

I agree with Zohner and check with a lawyer in your state. Also, the county assessor/register of deeds office may be helpful (or whatever your state calls the office that records land deeds and titles). Those are public records. Ask to see what the legal description of both properties are. Recently, a lot of it is online and may be mapped via GPS (at least in our state and county, all properties are now done this way). If not, the assessor's office should have a plat map and a legal description, hopefully with metes and bounds (rather than oak tree/etc.). So even if your MIL deed has ambiguous language, the other properties surrounding hers may have more exact language.

This guy sounds a little off the rocker so communicating via letter may be better than in person. But if he touches your MIL's fence, call the sheriff.

 We have the same law in Missouri,  Neighbor is SOL if he tried to pull that here.

zohnerfarms:

 If her ownership of the land on which the fence is placed has not been questioned for over 10 years, the land is hers by default, at least in Utah or in Idaho. I would suggest that you speak to a lawyer & lay out the facts above, & pay to have a letter sent to the neighbor.  He bought the land, knowing where the fence was, so he hasn't much ground on which to stand.  I suspect just the letter from a lawyer will solve the problem.

Yes a letter should do the trick. He bought the property as-is...knowing where the fence was.

 adverse postion .. she need to inform him about the property line .. she can get a copy of her survey at the court house sounds like he is gong to try to move his line ...

a Bully is hard to deal with please  stay on top of this he needs to remind that there are laws.

 Thank you, Gayla - Adverse possession is the term I could not remember.  If the fence line as been the undisputed "property line" for over 10 year, which it was before the neighbor moved in, your grandmother owns the land through "adverse possession".

I agree ... talk to a lawyer and have him send a letter.   But in the meantime, keep a close eye on your MIL's property; and if he touches the fence, be prepared to call the police.  I'd also suggest that for safety's sake, your MIL not deal with him alone (another good reason to have a lawyer ... she can refer everything to him/her.)

Most attorneys charge little or nothing for the initial consultation and having one send a letter outlining the legal status of the land may discourage further assertions of ownership. So it may be worth the $200 or so that would cost for your MIL's peace of mind.

Adverse possession. But, it may not work.  In Oregon, in any case, the possesion has to be disputed for those ten years.  The quickest way for a land owner to protect himself from a claim of adverse possession is to just give the adverse occupent permission to.

 So doubl;e check.

 In any case, take pictures of the fence from several angles so that you can have documentation of the current status.  Then, if you have old family picures that show the fence, save coppies of those too.  put them all together into a file so that if you have to go to court, you will have evidence of the fence and where it was/is and how long it's been there.