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From the Loans window, you want to click 'Open' next to 'Finish':
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This brings up the student's details as a Borrower:
(image)
Then click 'Current Loans' in the top bar:
(image)
Which brings you to a list of all their current loans:
(image)
The padlock 
Find their padlock in this list, and the combination is here:
(image)