Actually the law is often on their side, so it was likely not hard. Around here this happens all the time and so people are aware that gates need to be locked, signs posted etc, otherwise a Prescriptive Easement may be created : http://www.dirtlawyer.com/pe-summary.html IANAL but I have paid one in the past to get advice on this subject. ymmv, laws vary by jurisdiction, etc.

According to grandfather’s sibling[0], it may be that the crossing onto the other land became a subject of an adverse possession. Apparently in such a case you’re supposed to have a clear notice[1] on the property in question if you don’t want to lose it.

[0] https://news.ycombinator.com/item?id=11494002

[1] https://en.wikipedia.org/wiki/Adverse_possession#/media/File...