If they haven't done any morphology or interlinearization, then deleting the SpellingStatus.xml file should be sufficient.
If they have done morphology as well, then they will probably also have to delete the Lexicon.xml and WordAnalyses.xml files. Note that this will invalidate any interlinear data as well, so any Interlinear_{lang} folders will also have to be deleted.
And, of course, they should at least mark a point in project history before they do this. Maybe even create a separate backup to be safe.