Simply stated: this is a can of worms.

The person with the good well is almost guaranteed to end up with the short end of the stick.

Perhaps a good excuse would be "the local municipality won't allow it because of the risk of cross-contamination" or "my insurance company won't permit it due to liability reasons" will blow off the would-be moochers without creating undue hostility.

The person with the good well could still offer to fill a tank now and again to water gardens etc., to keep the good neighbour routine going.

Anyway, the best advice is: run, run away from this deal.