But out of curiosity.....will Messages have this working like iChat?
I've done some research, Screen Sharing seems to be hit or miss in Messages. I've seen setting the router UPnP, port forwarding, Buddy lists, and other hoops mentioned. Too much, it shouldn't be that hard. Something is broken at the moment and it seems it's been going on since the beginning of the year with no solid fix yet.
Try Team Viewer, no hoops. When you go to teamviewer.com they have a big button to install now, you can talk her through that, then the rest is a breeze. Install yours first then you can talk her through it. When you reach the screen for unattended access don't choose that, it sets TV to start with system start up, if you need that you can do it later in TV preferences.