There have been a lot of comments regarding CIBC vs BMO. I am aware that BMO made the top 100 on Maclean's list this year and CIBC did not.
But as within any large organization, when working in pocket environments, it's a toss up as to what the management is like. The only way to truly find out is to work there, but I'd recommend if she finds out which branch she would be working at first, and the physically go into the branch, go up to a teller and ask what it's like to work there.
As for the casual vs. the part time/full time; I'd say take the full time. The casual is a "we'll call you if we need you!" type job. If you decline too many, she would be dismissed.
Part time is something like a minimum of 20-25 hrs a week which is great provided that she can work in the evenings and/or weekends.
I can only speak for CIBC, it can be a great place to work, but again, it's what you make it. If you get lucky with good management as I have been in the past; you can have a lot of fun and see lots of advancement.
If you're unlucky...well...
Good Luck to your G/F!