My mom is going to visit me in Canada in summer, and we are going to US. Does she need a re-entry visa to return to canada after staying in US.
because she got a single entry visa to come to canada this time
A single-entry visa allows you to enter Canada only one time. Once you have left Canada, excluding travel to the United States and St. Pierre and Miquelon, you will need a new visa to travel back to and enter Canada. If your visa is still valid and you are travelling only and directly to the United States (including its Territories and Possessions) or St. Pierre and Miquelon, you do not need a new temporary resident visa to return to Canada. If your visa expires prior to returning to Canada from this area, and you are not in possession of a valid visitor record, work permit, study permit, or temporary resident permit (authorising re-entry), you will need to obtain a new temporary resident visa.