(Hell, my missus works for an water organisation who has witnessed a Welsh farmer who can dowse a water borehole for you with a better than chance hit rate, so really, what do I know!)
Sounds like he has had a bit of experience and maybe if he's a local he knows the water table and ground water which is rarely that well (pun intended ) mapped .
That's the old safe standby answer, what about if he didn't?