Sadly, if you have had a visa application denied before, it is very hard to get a new one - in any country. We don't have the full story of that case (only her side), but I am guessing that she may have known this and been less than honest when she was applying for her second visa about that denial. Still it's hard to know without confirmation.
Reading between the lines, she had her visa cancelled previously. She then seems to have flown from Canada to Mexico and applied at the border and I'm guessing this bit but probably filled the form wrong like not saying about the cancelled visa. Then the mucked up bit is rather than just refused entry she was put down for deportation but Mexico doesn't take non Mexican deportees so she ended up in the nasty holding system for illegal immigrants there. The US banned her from entering for five years so I think there must have been an error/deception on the application.
I guess the answer if you are Canadian is sort it out at the embassy or if you are chancing things do it at the Canada/US border not the Mexican one.
That sounds about right to me. This was also her third denial of the same kind of visa, and she was spending an awfully long time in Mexico for someone who is not a resident of Mexico. On further reading, it's possible that she was deported through this process because Mexico didn't want her back - most countries will look at you very differently if any other country has denied you a visa.
