Can a "friend" be a co-sponsor on a I-864 form? My long time friends' wife apparently does not have a high enough income to sponsor alone...

A friend can be a joint sponsor so long as he/she is at least 18 years of age, a lawful permanent resident or citizen of the U.S., and earns sufficient income or posses sufficient assets to meet the affidavit of support requirements. You would still be able to sponsor your spouse in the future assuming your income and/or assets are sufficient to sponsor two aliens. You can read more at  http://myattorneyusa.com/sponsoring-for-immigrant-visas-or-adjustment-of-status.