Do not force quit -- it will not help, and will likely make things worse.
LET IT RUN.
It may take 40 or even fifty hours.
I have never seen a repair not complete. Best to leave the computer alone. Make sure it is plugged in and not set to power-down.
If repair is successful, there is no reason to rebuild.
Your client is up against one of the strange limits of a huge database: the work can be done, but at a time scale that is inconvenient to the user. Splitting the Library will make these whole-database administrative tasks quicker, but at a significant cost in usability.