It is good advice never to hire a person who just arrives on your
doorstep. If you need work done, get advice and recommendations
from family and friends. Even then it's a good idea to get several
estimates before you decide whom to use.
Your aunt can check out a contractor's qualifications by calling
her local Better Business Bureau, or the consumer services branch
of her provincial government. There might be rules about the way
contracts are concluded outside of a business place and who can
make such contracts. It would not be unusual, for example, that
the contractor should be licensed and bonded in order to do business
in this way.
Consumer legislation in your aunt's province likely allows for
the reporting and investigating of such incidents. Her local provincial
office will be able to tell you what options exist.