Dave, you ask "Does the neighbor have any rights if he has been using this land?"
As an attorney I can answer and the answer is, it is indeed "LEGALLY POSSIBLE" in the future if the practice continues, but very difficult and subject to a lot of factual issues and time and state and local laws regarding "Adverse Posession" AND Riparian Rights (may include laws of certain easements) which is mostly based on "Common Law" although some jurisdictions may have codified it to some extent.
At Common Law in order to bring an action based on Adverse Possession the use must be Hostile,,,,,,,,,,Open,,,,,,,,,,Continuous,,,,,,,,,,Exclusive,,,,,,,,,,,,,,Adverse,,,,,,,,,,,,,,Notorious,,,,,,,,,and for a certain prescribed time period.
NOTE it is certainly possible to preclude one from acquiring legal rights under the doctrine of Adverse Possession, I advise that you (the neighbor) consult a local trained competent attorney for advice how to do so. Don't bet the farm on anything posted here, lay or professional.
NOTE I'm NOTTTTTTTT saying the neighbor has acquired any rights as of yet whatsoever IM ONLY SAYING that under the law in many jurisdictions such IS LEGALLY POSSIBLE (and/or the related doctrine of acquired easements) for the neighbor to over time subject to local laws and facts and time and necessity and a whole lot of other factors, acquire some rights.
IMPORTANT my professional legal advice to you is DO NOT make any important decisions or take advice based on non attorneys or even attorneys here (including me) unless and until all state and local laws, both common and statutory, have been thoroughly researched by a competent trained professional and all the facts determined........
ALSO consult a competent trained professional attorney (NOT Billy Bob or Bubba or the know it all brother in law) who specializes in riparian rights as there are strict laws concerning changing the natural flow of water and drainage ITS OFTEN NOT LEGAL to make changes !!!!!!
John T BSEE, JD Attorney at Law