Pre September last year, your mum would most likely not have been asked for proof of citizenshp but I would not risk that these days. It's usually not a problem going into Canada but re-entering the U.S. where I.N.S. is a lot stricter.

see:
http://www.ins.gov/graphics/hditravelcanada.htm
http://www.ins.gov/graphics/howdoi/legadmit.htm

There must be some register in her home town where she can obtain some proof?

http://travel.state.gov/vital_records_services.html

Voter registration card is insufficient proof these days.