@jespah,
I know she only uses her phone. Out of all my messages with lots of people only hers updates.
I remembered I have two accounts for Facebook (long story) and had a play myself.
Even when the message is viewed many times or if things are typed and then deleted and using more than one device the 'seen' doesn't update.
The only way I've got it to update is to mark it as unread and then read it again.
It begs the questions why she would be doing that, I'm gonna keep playing with it, it's got me very curious.