If she was only a student from January to March, she was not a full time student for 2019, for purposes of claiming her as a dependent* under the Qualifying Child rules.
For purposes of her claiming the refundable portion of the American Opportunity Credit, on her own return, , she was not a full time student for 2019.
For purposes of you or her claiming a tuition credit, she was a half time or more student for Jan.-Mar and tuition paid for that counts for the credit. She does not have to be full time.
*There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status test, a relationship test and a residence test. Only a QC qualifies a taxpayer for the Earned Income Credit and the Child Tax Credit. The Other dependent (qualifying relative) credit is worth (up to) $500 per dependent and is non-refundable. That is, it can only be used to reduce an actual tax liability.
A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:
- Closely Related OR live with the taxpayer ALL year (not even one night at the non-custodial parent’s home).
- His/her gross taxable income for the year must be less than $4200 ($4150 in 2018)
- The taxpayer must have provided more than 1/2 his support
In either case:
- He must be a US citizen or resident of the US, Canada or Mexico
- He must not file a joint return with his spouse or be claiming a dependent of his own
- He must not be the qualifying child of another taxpayer
