If two people own a home, mtg is $340,000, onc person wants out, wants the other person to buy out, what would they be entitled?

Mother & daughter situation, right to survivorship. mother wants out, thinks she is entitled to half of the house $170,000, however the mortgage of $340,000 is to the max on the house, and the value of house is less than the mortgage giving no equity. Is she entitled to $170,000.
How does one buy someone out, is it the equity or the mortgage???

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