I know how grandmas are and she may not be willing to file a police report (which is probably what he is counting on). Talk to her seriously tell the collectors to only contact you grandma by mail. Dispute the credit bureaus as fraud, file the Fraud affidavit and see if grandma would be willig to file a report. He has opened her up to being sued for whatever else he may have done and its best to **bleep** it before it can get worse.
Have eyou spoken with the original creditors and not just the CAs about it? Im sure that they will understand the logic. What would a 90 year old woman need with two cell phones? Hopefully he didn't talk her into signing anything. IF nothing else take him to small claims court and get a judgement against him. This will probablly hold up in a fraud claim.