Hi Kooky,
When was the first CAS issued? Before 23 July? If so she can study bellow level B2. If not her level of study should be B2 or above.
Whats the level the new college offers? B2? if so you can make a fresh application. There wont be any problem.
How did the new college assessed her? Even though secure language test is not mandatory for english courses I would say it would be better if your sister can prove she is competent in level B1 with a results in secured english langauge test.
Apply with the new CAS letter if this is level B2. In the mean time ask your sister to sit one of the secured langauge test. If she passes B1 level then she can send the result as a further evidence/ additional information.
secured english langaue test is explained here.
http://www.ukba.homeoffice.gov.uk/sitec ... estst4.pdf
if you have any queries let me know.