According to the government, a financial joint sponsor can be anyone who meets the government's income requirements and is willing to take financial responsibility for the spouse seeking a green card. This financial co-sponsor does not need to be a family member or relative of either the sponsoring spouse or the spouse seeking a green card.
However, the financial co-sponsor must be a U.S. citizen or U.S. green card holder, at least 18 years old, and living in the United States.
You can find more information about the government’s income requirements here.
For more information about the financial sponsorship form (the “Affidavit of Support”), check out this guide.