She really can not take the deduction unless she pays part of the home office expenses to you. You are the one paying the expenses so you get to deduct that portion of the home expenses that relate to your office from your income. Her income does not have any office expenses offsetting it. You could make her office part of your office and rent her the space at market value. You would recognize her rent to you as income and offset the combined offices on your tax return and she could take the cost of her portion of the space (what she pays you) as rent expense thus reducing her taxable income. Sounds convoluted but it is your house and your expenses not hers.
