Adjustment of Status - I-485

I filed my mom's I 485 recently. Now USCIS mailed a REF letter:

** Submit all supporting tax documentation (W-2s, 1099s, Form 2555, and all supporting tax schedules) submitted to the IRS for the most receint tax year. The joint sponsor must submit all supporting tax documents for the most recent tax year.

** Based on the documents submitted with Form 1-864, Affidavit of Support, for the joint sponsor, the income did not meet 125% of the poverty guideline for the joint sponsor's household size.
Submit evidence of assets that meet the standards listed below or obtain a different joint sponsor.
Assets must equal the stated difference between the joint sponsor's household income and 125% of the poverty guideline.........
Note: The total income (line 22) on IRS Form 1040 is used to determine qualifying income for a sponsor, not the "Gross Receipts" (line 1) from IRS Schedule C or C-EZ.

I am a college student and work as part time, my income is not good enough to sponsor her application, so we have a joint sponsor in the I 864. The joint sponsor household is 3, plus my mom should be 4 and her income in 2016 W2 is $33924.59 which is satisfied. However, when we submit the application, the joint sponsor didn't file 2016 tax return yet. Does the above statement from USCIS is requesting the joint sponsor 2016 tax return? If really confusing me as they told in the second statement that the joint sponsor did not meet 125% of the poverty guideline but the first statement looks like need more tax document?? Need some advise please......