There is no definite answer to this question as there are no published numbers.
If you are sponsoring the trip, then you should have enough balance to cover her expenses like travel and enough inflow of funds (i.e. salary) for recurring expenses like lodging etc.
She should have enough funds to show ties w/ home country. Also, don’t try to move around too many funds now. Carry 3 months bank statements to show that funds weren’t gathered from different accounts just for the purpose of interview.