sounds to me like she's just being a little so-and-so. I used to have dozen of buttons and most of them lived happily in a group setting but every once in a while I'd get one that would develop an attitude overnight. I would ask myself the following questions
1. do these birds have a big enough cage
2. does this cage offer enough "spaces" and sight barriers to let the birds get out of sight from one another
3. Has anything in the environment changed recently when this agression started - ie the birds moved to a different location, change of anything in the cage, separated from another group of birds, etc etc.
Right now I would add another food and water dish so they don't have to share resources. It sounds like she is just posessive of the food and doesn't really have a hate on for him in general. Is the food available all the time or does it run out during the day? What kind of cage are they in?