Tell her that she has to get the employer's permission before taking/consuming any of the employer's belonings----food definitely is one of them.
For me, I sometimes will specify that she can take 'some of the said food' if she wants to, meaning those I didn't mention, please just don't touch.
Emphasize the Chinese saying to her:
不問自取,視為賊也.
But dear xiatian, I've got problems too, after contract renewal. :-( :-(