If your girlfriend is also a Canadian citizen, all she has to is come here. If she is not a citizen, she will have to get a visa or eTA (depending on her citizenship) and come here. It’s much more complicated if she’s not a citizen.
Basically, she needs to live in Canada (or another country with similar divorce rules) for long enough to divorce her husband. In Canada, she needs to be here a least a year before a court will take the separation seriously. (Though a divorce lawyer may be able to tell you of other ways she can apply for divorce, than just waiting.) If she is not a citizen, she cannot stay here as a visitor for a year straight – she has to leave after six months, though she can return immediately.