The I-864 asks for the income as listed on the last 3 years of tax returns if required to file. If not required to file, she can indicate that on the form. She must submit the most recent tax year documents. Current income is king. If she can document that her CURRENT income is sufficient, she should be fine. Current annual income is calculated as follows:
Gross income from most recent pay stub multiplied times the number of pay periods per 12 months. Also, supplemental income can be used.