Anybody can apply for a visit visa but the odds of getting a visa may be increased if they have a person in Canada who is inviting them. If their problem is lack of funds, it would help if the person in Canada writes a letter stating that they will vouch for them as well as showing some bank statements or pay slips.
It does however have to be clear that it is only a visit. In some cases, a relationship to a person in Canada can hurt your chances of getting a visit visa, mainly if it is a spouse or a fiance. In those cases, immigration often refuses to believe that the person only wants to visit and believes they want to stay on.