Get TeamViewer, it's free for non-commercial use. You both install it, launch it, she gives you her ID and password. It's pretty easy to use.
All depending on the hardware combo I have found the recent version to have some quirks sometimes with double-clicks, but that's always accomplished by Command-O. You can set it to start at system startup if you need to do restarts and want access right after the restart. It always gives the same ID number when it starts but generates a new password. You can also lock in a secondary password for unattended access.
It gets through any router/modem, no need to mess with port forwarding or anything.
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.
Glad it worked out for you. I use it to connect to Windows machines, and a wide range of OS X from 10.4 to 10.8. I don't know why Apple with all of its money can't make something just as compatible. They do know what's best for us I guess. LOL.
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