According to TSA:
Originally Posted by http://www.tsa.gov/travelers/airtravel/acceptable_documents.shtm
Passengers who do not or cannot present an acceptable ID will have to provide information to the Transportation Security Officer performing Travel Document Checking duties in order to verify their identity. Passengers who are cleared through this process may be subject to additional screening. Passengers whose identity cannot be verified by TSA may not be allowed to enter the screening checkpoint or onto an airplane.
Strictly speaking, this means your mother-in-law cannot present an acceptable ID, since she doesn't have one, and therefore is eligible for the alternate procedures. Reports here indicate that these procedures are time-consuming, so she may want to allow plenty of extra time to get through security (and expect lots of personal questions as a part of that verification process).
This, of course, assumes that the TSOs on duty at your mother-in-law's airport understand the rules as well as TSA does. As always, your mileage (and hers) may vary.