It means both "he will" and "he won't" depending on context, but this course frequently gives the mutated form without context. How are we meant to tell which one it is without context?
If answer required is "she will not" the should the question not be "Fydd hi ddim"?
I'm wondering the same as JasonAylett and tachwedd. Surely there should be a ddim there somewhere? Or is this just a casual, slang sort of thing to say?