Thanks @jimr1 ,
That post has no accepted answer yet on stack, and seems to refer to the generic growth of a table. I think what our user was dealing with is rather a bug or something, but either way, he seems to have reverted to stack and gotten an accepted answer here:
To me it looks like this sequence might do the job, provided you still have control over your database:
ALTER UNDO TABLESPACE tablespace_name SET INACTIVE;
DROP UNDO TABLESPACE tablespace_name;
SET GLOBAL innodb_undo_log_truncate=ON;