Cpok, have you seen my response to another topic?
http://www.canadavisa.com/canada-immigration-discussion-board/can-someone-post-t12304.0.html I'm not a specialist, but I would change some parts on your letter... For example: you said you will cover accommodation and medical costs, but what about the other ones, such as food?
Probably you guys had that idea already, but I would also ask for a letter from her boss, stating that she has only that period of time to travel and must come back after that short trip. Does she study as well? If yes, something proving that too would be nice.
I have read your other topic about how difficult it is for russian girls to get a visa. I have friends from Ukraine and Russia, and they tell me the same... It couldn't be different with girls from latin america too!

I have been waiting them to say "denied" or "approved" since two weeks ago (am trying the canadian visa for the second time - 1st time approved! but expired since june). I think as long as she got the american visa twice and didn't overstay, and has a job, she may get her visa approved. But again, I'm not a specialist, it's just a thought.