If she is a full time student and under the age of 24 then her income is not a factor in determining her eligibility to be claimed as a dependent under the Qualifying Child rules. However, it can be a factor to determine if she is providing over one-half of her own support. See this IRS website for support calculations - https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
Being claimed as a dependent means she is not eligible for a stimulus payment.
The other rule is that if she CAN be claimed as a dependent, even if not being claimed, she must indicate on her tax return that she can be claimed as a dependent.
To be a Qualifying Child -
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
