There are different types of test she can take, i.e. IELTS, TOEIC, etc. that doesn't require enrolling in any course.. take a look at this list .. http://www.ukba.homeoffice.gov.uk/sitecontent/applicationforms/new-approved-english-tests.pdf

Also, your wife must have passed the Life in the UK test aside from getting a B1 level mark in any English test listed in that link above.