Also , your GREAT mistake was your advice for her to quite job. It could be a strong tie (togetehr wioth a shorter visit, like "on summer holiday" for 3 weeks.
If she gets visa she HAS to have a return ticket with a date of flight 3 weeks from entry. (Buy one with option of changing the date) .
Also, if by chance she will not get at POE a hand written due date to leave, then she may stay for 6 months and apply for extention 30 days before the end of stay.
However, if she gets that "date of leaving" in her passport and it is only 21 days , then you still could apply for extention ; HOWEVER, applying for extention so soon (within 21 days) will couse CIC to suspect that when she asked for a visa for 3 weeks visit she had been lying, that applying for 3 weeks visa she kept in mind 6 months stay, and it is a misrepresentation.