Yes, ask the joint sponsors wife to fill out the form I-864A. There may be other documents that are missing, such as the form 1099s and 1040s, but this is unclear. If the documentation is incomplete, then this can be another reason for future delay.
The petitioning husband can be asked why he or she did not file a tax return. If so, then the petitioner will have to prove why it was unnecessary as a matter of law to the satisfaction of the USCIS.
There are serious legally enforceable financial obligations when joint sponsors sign forms I-864 and I-864A. These I-864 obligations can last ten years unless you become a U.S. Citizen by naturalization sooner. Becoming a naturalized U.S. Citizen as soon as legally possible is a courteous thing to do, where the joint sponsor makes such an unselfish gesture. However, this is a personal decision and an applicant must study and remain legally eligible to file for naturalization, as well.
This is general information and is not meant to create an attorney-client relationship.