Sharing between 2 Macs...

Several options:

1) Store all documents on a cloud service, dropbox/box/google drive/ microsoft one drive. Then they would automatically be accessible in both places

2) Store all documents on an external drive and transport it front site to site.

3) Use a sync program (e.g. Chronosync) to sync the documents folders on each machine (She should be able to use Back to My Mac to access each machine from the other).

 

It is times like these that I wish Apple would include some sort of utility for creating mobile user accounts without the need for OS X server. Mobile accounts allow you to place a user's home directory on an external drive, like a flash drive or hard drive. When you plug the drive into another mac, the user account makes itself available for login. All the user's data is stored on the external drive and can be encrypted with filevault. A very elegant solution for situations like this.

 

Dropbox is fine, I have my wife's MacBook Air and iMac set up much as you describe. She doesn't use her Desktop for file storage but this article describes a way to sync desktop via Dropbox.

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That's an option. It's not like Server is terribly expensive, it's only $19.99

 

You might want to look at this : http://www.zibity.com/macdropany

It allows you to sync any folder via dropbox or some other cloud services supported.
And its free!

I use it works as advertised.

 

You can do that too. if you explain to her to keep all the data she wants to share in the dropbox folder.
The more elegant way of doing it is with the software tool i mentioned above.

Above program is a gui (graphical user interface), it creates the symbolic links for sharing folders via dropbox without having to move the folder structure. (http://hints.macworld.com/article.php?story=20120803093247391)
For users who are not familiar using terminal commands.

For example you can share the documents folder (incl. subfolders) between two iMac's using Dropbox, without having to move any of the folders/sub-folders. (subject to dropbox size limitations -> money)

This is transparent to the user and typically the easiest way to achieve what you wanted.
It is very easy to do.

If you want to know more google: symbolic links os x dropbox


Another thing she might want to do is leave her iMac in the remote location on standby. Using "back to my Mac" she can see her remote iMac in finder and get to her files in case she forgot something or even use screen sharing to remotely control that iMac. (http://support.apple.com/kb/PH13863?viewlocale=en_US&locale=en_US)
http://support.apple.com/kb/PH14179
google: setup back to my mac