Eminent domain seems like it may be a stretch, but there are other doctrines that may be applicable. One doctrine is adverse possession, and another is prescriptive easement. There are a number of elements to them, and it is impossible to tell from this scenario if they apply. So, I think it would be worthwhile to consult with an attorney at this time. Regardless of whether those doctrines are winnable arguments for your neighbor, this is the kind of situation where good lawyers can be very helpful in resolving the situation to your satisfaction.
