The mother's discretionary income does not matter when looking at the support test.
Food, clothing, shelter, education, medical and dental care, recreation, and transportation make up support amount. The amount she saves, gives to charity, gifts the the kids and grandkids, etc., are not considered in determining her support amount.
The amount of support for shelter costs takes into account the fair rental value of the lodging, including a reasonable allowance for the use of appliances, utilities, and furniture, not the actual amount spent. So if it would cost the mother $1,000 a month to rent a furnished apartment equivalent to what the daughter is providing, that is the amount used for the 50% support test.