A granted prescriptive easement will likely allow for the the maintenance and replacement as necessary.1. If she's able to get a prescriptive easement, would that allow her to only use the land for the existing septic leech field, or would it allow my neighbor to put a new one in to replace the existing one when it fails. It was installed in 1971 and has got to be close to the end of it's useful life.
2. If after I purchase the parcel, is it legally possible for me to grant her easement for the existing field only and not for any future fields which have to be put in to replace this one when it malfunctions?
Possibly. But if she can obtain a prescriptive easement, she will have the right to to enter and maintain her property.
3. What if she sells her house? Will the next owner retain the easement?
The new owner can obtain whatever rights the grantor owns, based on the language of the granting document.
As I suggested earlier, you don't want to enter into any transaction such as this without legal representation.

