Is your husband able and willing to put up a "public charge bond" on his mother's behalf? This would be in addition to an I-134, Affidavit of Support. She would need to request it as an option from the Consular Officer. IF agreeable, the ConOff would need to put a request in writing for her to give to her son who would have to contact DHS. The minimum is $1,000 and goes up from there.
INA 221 Issuance of Visas. [Substitute Secretary of Homeland Security for Attorney General]
(g) No visa or other documentation shall be issued to an alien if (1) it appears to the consular officer, from statements in the application, or in the papers submitted therewith, that such alien is ineligible to receive a visa or such other documentation under section 212, or any other provision of law, (2) the application fails to comply with the provisions of this Act, or the regulations issued thereunder, or (3) the consular officer knows or has reason to believe that such alien is ineligible to receive a visa or such other documentation under section 212, or any other provision of law:
Provided, That a visa or other documentation may be issued to an alien who is within the purview of section 212(a)(4) , if such alien is otherwise entitled to receive a visa or other documentation, upon receipt of notice by the consular officer from the Attorney General of the giving of a bond or undertaking providing indemnity as in the case of aliens admitted under section 213: Provided further, That a visa may be issued to an alien defined in section 101(a)(15) (B) or (F) , if such alien is otherwise entitled to receive a visa, upon receipt of a notice by the consular officer from the Attorney General of the
giving of a bond with sufficient surety in such sum and containing such conditions as the consular officer shall prescribe, to insure that at the expiration of the time for which such alien has been admitted by the Attorney General, as provided in section 214(a) , or upon failure to maintain the status under which he was admitted, or to maintain any status subsequently acquired under section 248 of the Act, such alien will depart from the United States.
8 CFR § 213.1 Admission under bond or cash deposit.
The district director having jurisdiction over the intended place of residence of an alien may accept a public charge bond prior to the issuance of an immigrant visa to the alien upon receipt of a request directly from a United States consular officer or upon presentation by an interested person of a notification from the consular officer requiring such a bond. Upon acceptance of such a bond, the district director shall notify the U.S. consular officer who requested the bond, giving the date and place of acceptance and the amount of the bond. The district director having jurisdiction over the place where the examination for admission is being conducted or the special inquiry officer to whom the case is referred may exercise the authority contained in section 213 of the Act. All bonds and agreements covering cash deposits given as a condition of admission of an alien under section 213 of the Act shall be executed on Form I–352 and shall be in the sum of not less than $1,000. The officer accepting such deposit shall give his receipt therefor on Form I–305. For procedures relating to bond riders, acceptable sureties, cancellation or breaching of bonds, see §103.6 of this chapter.
Form I-352: http://www.ice.gov/doclib/news/library/forms/pdf/i352.pdf
Page 5, item (G)(2) BOND THAT ALIEN SHALL NOT BECOME A PUBLIC CHARGE.