Provided there are no changes made by the client, that should work.  

HoneyRoadBooks said:

Would I do a backup of my Sage to a USB, and then when I get to his office do a restore from file?

Yes.

If your client is able to restore a backup, you could keep backups available to the client in some software such as DropBox.  (then they will not have to pester you for the updates).   The client could scan documents and place them in the DropBox folder for you.

There is no feasible way to automatically merge two sets of Sage 50 books, get it in writing from the client that if they understand that if they are to enter any data, or update an address, etc. it will be overwritten with your updates.

And you may need to have insurance to do work for clients, etc.

I am not really sure how tech savvy this client is. I take him to not be much for technology so to use DropBox might be a problem. Just don't have much experience with "save a copy" under the file menu. Wanted to make sure that maybe wasn't a better option than doing backup and restores?

Backup and Restore takes a bit longer, but the backup archive can't be opened and modified, so you know it's a full copy, and it hasn't been changed.

Save a Copy will make a full copy, but there is the possibility of a missing file, and it's just too easy to copy an old file overtop the new one, accidentally.

Why not just setup remote desktop and log into your new client's computer. There are plenty of low cost services on the market which allow users to share desktops or use MicroSoft free RDP