Would like to know the reasons why a visitor status may not be renewed.

[bisonette]

The g/f of my ex is looking to come to Canada next year on a TRV. I have been told that if she and my ex get married during the time she is here, she will need to apply for her PR if she is to stay here for good. It was mentioned that she will need to renew her visitor status in order for her to extend her stay here while her PR app is being processed.
What I would like to know is under what circumstances, or reasons, would her visitor extension be denied, thus meaning she would have to leave and go back to Thailand until her PR app has been approved.