from a legal standpoint, there is nothing they can do. they may allege that they have an easement by prescription from the previous owner, but this is highly, highly unlikely (See: the last driveway post). if anything, just tell “a” that they encroach on your area and you want them gone. if they are paying him, he’ll tell them to be more careful. if they’re not, he’ll either get annoyed at them or tell them to stop for good. mumbling something about a lawyer may help your cause. if i understand you situation correctly, it doesn’t sound like the “other neighbor” has any standing to be upset, though it’s not like that has ever stopped anybody.
this is why having to interact with other humans is annoying.