Both you and your friend can stop the process by withdrawing your affidavits of support. This can only be done if the green card application is still pending. If his green card application is already approved, neither you nor your friend can withdraw the affidavit of support. You will neeed to contact USCIS to withdraw the affidavit of support. You can read more at http://myattorneyusa.com/family-immigration.
Answered on Jan 16th, 2017 at 4:01 AM