First, recovering the student's progress: In my experience it is difficult to completely delete content. The student responses may show up when you view the quiz in speedgrader, and if not, canvas support should be able to offer more information about how to recover them.
If you are a teacher in your collegue's course, you can copy the quiz using the course copy options of import course content (in the right side menu of the course settings). Just select "copy specific content" so that you only copy the quiz. If the quiz does not repeat a quiz name in your current course, it should not write over the existing quiz. I had something similar happen and resolved it with a very subtle change to the quiz title in the course I was copying from (added spaces or punctuation work) before the second time I imported the quiz.
For the future, before you alter the quiz, use the three dot menu on the quizzes or assignments tab and select "duplicate." Give the duplicate its own name, and then you should be able to alter each copy in different ways.
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