If her $2700 work income was reported on a form 1099, rather than a W-2, there will be self employment tax on line 24 (form 1040). This is separate from income tax (line16).
"Self employment tax" (SET) is how you pay social security & Medicare tax, on earned income when none was withheld. SET is due even if you do not have enough income to have to pay any income tax. The threshold for having to file a tax return, when you have a 1099 is a mere $400.
