If she is granted social services, whomever sponsored her green card is liable for the money.
Probably not actually.
While the I-864 is enforced more and more, it appears the I-134 is rarely enforced even though technically it can be enforced. But that might be moot anyway, since Medicaid doesn't necessarily make you a public charge.
Public charge from USCIS: http://www.uscis.gov/green-card/green-card-processes-and-procedures/public-charge
The next four explain why the I-134 is considered "not enforceable".
http://www.alllaw.com/articles/nolo/us-immigration/i-134-affidavit-support-instructions-tips-sponsors.html
http://www.ilw.com/articles/2006,0110-wheeler.shtm
http://www.capriotti.com/pubcharg.htm
http://www.nolo.com/legal-encyclopedia/filling-out-form-i-134-affidavit-support-help-us-visa-applicant.html
Now. I'm very curious about something.
The OP submitted I-134.. less than 5 years ago? And hasn't needed to submit the I-864 later? How and when did adjudication take place?
A timeline would be good.