As you say, you don't want to inherit such a problem. It's up to the seller to clear up problems such as this.
Since the seller seems to be previously involved with both the neighbor's parcel and the associated septic leach field, the seller needs to straighten this out.
There is not much to go on here. It would be important to know the history of how the lot was sold to the neighbor and what additional rights, if any, were conveyed.
It is unlikely that the neighbor has an adverse possession claim simply from the maintenance of the leach bed. A prescriptive easement is possible.
You will certainly need to have legal representation in this transaction, even if it appears to you that the seller has cleared up the problem.
That's what it sounds like.It sounds to me like they are merely threatening court action so that they can get an easement granted, or else they would simply seek the court action in the first place.

