In case you are interested, here are before and after pictures and information about a vintage Lexington Briefcase that I rehabbed. It was black and dark bags are generally easier to rehab then lighter bags, but it did have condition issues; including damaged piping and verdigris.
https://forum.purseblog.com/threads/coach-rehab-and-rescue-club.833400/page-970#post-31594799
I don't see a
coach hangtag on your sister's briefcase, is it missing? You can call coach customer service and they will send a free replacement hangtag. They will ask the style number for the bag and the color of the bag and the hardware. They usually send a simple, metal hangtag that matches the hardware.
If you think that the turnlock is damaged, they will also send free turnlock replacement kits for vintage bags.
Good luck!