Been my experience that courses that are used seldom reduce in size ... they only grow over time.
' ... was imported from 1.9 when their moodle went from 1.9 to 2.2, with many problems.'
That could be true, for sure. Problems that existed don't necessairly get automagically fixed. Now it depends upon how 'valuable' that course was to justify the manual work that might need be done to restore it.
Have you unzipped the 1.9 backup file locally and checked the moodle.xml file to see just what version of 1.9 this backup file comes from?
Does the moodle.xml file also include roles?
Did the backup contain an add-on module no longer supported in 2 ... like journal?
One can reduce the size of a 1.9 backup by manually editing the moodle.xml file and removing tags that reference files found in course_files.
Move the course_files directory out of the work space used for inspecting the unzipped backup. One could then re-zip the backup and use the reduced size zip to restore.
All the links to files, of course, will be busted, but once the course is established, create a repo for it and then by means other than Moodle UI (scp/ftp/mount, etc.) copy the contents of course_files to that course repo. Now the teacher has all the files that were in the course before and they must re-link them. Yes, that could be a lot of work also, but NO LEGACY.
Think I'd try that IF it was a 'valueable course'. ;)
'spirit of sharing', Ken