"She owns that?"
Translation:Hi sy biau hwnna?
in another situation would 'mae hi'n biau hynny' work? i understand that isn't the form being taught in this instance.
biau/piau is a defective verb that does not need another verb such as mae to make it work in the present tense. In the sentence given, for example, the sy is not actually needed, although people do tend to include it informal speech.
So in your example the mae is not needed and is not used in practice. 'Hi biau hwnna' is fine on its own.