It is harder these days to cross the border with just a driver's license, in either direction. It depends partly on how you are travelling - for example, Horizon/Alaska Airlines would not let my boyfriend on a flight to the US (we are Canadian) with just his driver's license, as it alone is not proof of citizenship. However, the Victoria Clipper web site advises that US citizens must have at least a valid driver's license for entry into Canada, and a passport is recommended.

There is a discretionary factor to the Customs and Immigration process, however. An individual officer may decide that a DL is sufficient. Or not. The government of Canada Immigration Officer's manual states this is the process to be followed with US citizens visiting Canada:


6.5.1 U.S. citizens
The U.S. passport as well as the Certificates of Citizenship and Naturalization are
considered prima facie evidence and are acceptable proof of U.S. citizenship.
The birth certificate and the U.S. voter’s registration card, when accompanied by
another document bearing a picture of the holder, are considered indicators and may
be an acceptable proof of U.S. citizenship.
The U.S. military identification card, although a good supporting document, is not prima
facie evidence of U.S. citizenship; you do not have to be an american citizen to be in
the military.
Sometimes a verbal declaration will be sufficient to satisfy you that a person is an
American citizen. In the end, it is up to you to decide whether a verbal declaration or,
when deemed necessary, which documents will satisfy you as to a person’s claim to
citizenship. For example the following documents, driver’s license- health card-school
records-credit card, although not prima facie evidence, have been used along with a
verbal declaration to satisfy an officer that a person was an American citizen.
To assist the travel industry, airlines and travel agents have been supplied with the
following information:
a) a U.S. passport is the ideal identification for U.S. citizens travelling to Canada,
b) U.S. citizens may travel to Canada without passports if they have other means of
establishing their citizenship, such as a U.S. birth certificate or naturalization papers,
c) U.S. citizens travelling directly to Canada from the U.S. may be able to satisfy
claims to U.S. citizenship by presenting identification documents such as a U.S.
voter’s registration card, medical card, credit card or educational records, and one
other identification card containing the holder’s photograph, such as a driver’s licence.
Diplomatic, consular, official and service passports meet the passport requirements for
visitors.

(From www.cic.gc.ca - look under Publications, Manuals, POE, Chapter 1)

If your mom is going to be doing some other travelling in the future, it might be worth while to track down a birth certificate and obtain a passport for her. There must be a process to do so...

Else, I would advise that she bring along something other than her driver's license, maybe a voter registration card or property tax statement or social security info. The officer may be satisfied with her driver's license, but if not, she will at least have some supporting documents.

And if you are flying, make sure that you know what ID the airline will require! My boyfriend would not have made it on that flight if he had not been able to produce further documentation of citizenship.