The criteria for Qualifying Widow is detailed here:
One of the criteria is that you have a "Dependant Child"
must have a dependent child in order to file as a Qualifying Widow or Widower
A dependant child is defined here:
And they must be below the age of 19, or 24 if a student, or permanently disabled:
Age: They must be one of the following:
Under the age of 19 on the last day of the year and younger than you (and your spouse if filing jointly)
A full-time student under the age of 24 on the last day of the year and younger than you (and your spouse if filing jointly)
Permanently disabled at any time during the year, regardless of their age
Since the daughter is above the age of 24, she will only qualify if she is permanently disabled. Otherwise she will not be considered a Qualifying Child, and the mother cannot file as a Qualifying Widow.
However, if the daughter earns less than $4200 a year, the mother can file as Head of Household, with the daughter as a Qualifying Relative, as long as other conditions are met too.