Can I reduce the size of the Time Machine data base

On an old 2008 iMac running 10.5.8, my colleague has a business that relies on her computer six days a week. She has Time Machine outputting to a 500 GB external drive, yet her internal drive contains only 120 GB. The Time Machine data base goes back to 2008 and occupies over 400 GB.

I would like to partition the external drive to hold a copy of her system (now on the internal drive), for use in case the internal drive fails. However, the Time Machine data base is too big. Is there a way to prune the data base to make it smaller? She would like to keep some backups back to 2008 if that is practical.

I see that the data base is made up of dozens of dated files. I'm tempted to simply delete a bunch of them, but I have read on these pages here that Time Machine gets fussy about that. I'm afraid that deleting a bunch of files would ruin her backups back to 2008.

What would you suggest?

--Gil