How do I lock down a non-geeks MacBook
Some things I would look into in such a scenario:

Backup

Absolutely essential, and something you can set up easily. At least attach an external drive and configure TimeMachine. If you want to add more protection against handling errors or such, use a service like Backblaze or Arq to get an off-site backup.

Remote Access

Sooner or later you will do support over phone and it is much faster and easier to do if you can see what is happening on the screen. I usually use Teamviewer which is easy to setup and use. Just make sure that everything is working as it should before you leave house.

Restricting things on the computer itself

With remote access in place you can get rather restrictive with what the user can do. You probably don't want to give them administrator rights (so make sure you set up a non-admin account for them) which eliminates most handling risks. I would also restrict application installation to the App Store.