I would recommend Quebec City rather than Montreal due to too much English being spoken in the city. In fact, Montreal is bilingual enough to get by without much French. On the other hand is Quebec City is overwhemlingly Francophone (French Speaking). Outside the tourist areas, it will be essential for her to learn French to communicate with the locals.
Hence, look up the Laval University in Quebec City. I know for a fact that they have French immersion programs with affordable homestays. The URL to the French Language Program for non-French Speakers is
http://www.elul.ulaval.ca/sgc/lang/en_CA/pid/9070