On the lower end of the rent scale, no. If someone is renting a basement suite from me and paying 40 - 50% of their income I am okay with that. There is not really anything cheaper on the market and they need to live somewhere. If they don't have high debt in other areas they will find a way to make it work.
To answer your other question, CMHC considers a household to be in core housing need if its housing does not meet one or more of the standards for adequacy, suitability, or affordability and it cannot access acceptable local market housing without spending 30% or more of its before-tax income on shelter. In Edmonton, this is approximately 50,000 households.
t already written, 30% is a reasonable amount that tenants should spend on rent. However, you should base not only on gross monthly income of your applicant. Do online research in your area. Look for similar properties on rental websites and see what price landlords charge for their rentals. In the USA it could be Zillow, Rentberry, Craigslist.