First of all, he should not just be looking at desal.
Second, he would be looking at a long and rewarding profession. IMHO one of the best.
Have him check with the AWWA:
Operator Assistance - Professional and Technical Resources - AWWA
and the California Department of Health covers Licensing for water treatment:
He should also consider a water distribution system license as well.
Drinking Water Treatment & Distribution System Operators
BUT he should also not rule out wastewater.
State Water Resources Control Board
Good luck to him it is a very rewarding career. The best water engineers I ever met, were water treatment operators.