Is it possible he means doing this on dosage data? I find that --hwe does not work if dosage format is specified. For example:
plink --dosage dosage.dat format=1 --fam dosage.fam --map dosage.map --hardy
or
plink --dosage dosage.dat format=1 --fam dosage.fam --map dosage.map --hwe 0.00001
Neither of these seem to work for me.