The rear brake is a useful tool, and does help you stop faster if you know how to use it. Therefore, you should learn how to use it. Your advice to her is poor. She should take a course or two, to get good instruction (probably wouldn't hurt you to do it, either). The advanced MSF course includes practice with the rear brake, including controlling your bike if you do lock it up.
1993 Ducati M900 Monster "Patina" (228,000 miles, so far) -- 1995 Ducati M900 (wife's bike) -- 1972 Honda CB450 (daughter's bike) -- 1979 Vespa P200 (daughter's scoot) -- 1967 Alfa Romeo GT Jr. (1300cc) -- 1964 Vespa GS160 (160cc 2-stroke) -- 1962 Maicoletta scooter (275cc 2-stroke) -- 1960 Heinkel Tourist 103A1 scooter "Elroy" (175cc 4-stroke)