X is really excited about getting into video games, but she tends to forget to move when attacking or to attack when moving.. also (a huge problem in isometric games like Bastion) she tends to only press one WASD key at at time, moving in straight lines rather than continuous paths.
What would be a good method or game to teach her to rest her fingers on the WASD keys and use several at a time, while also not forgetting to attack?
It's a matter of practice.
I think there are 3 genres which are very appropriate to play in order to learn movement/mouse coordination.
They rely heavily on positioning and simultanious attacking (using the mouse) so I think these would be the perfect games to practise with.
It does not mean you are limited to these genres.
Another way would be to watch her play and notice what she should improve at. After giving tips, let her try out the tips if possible even if there is no objective nearby.
Giving a reason for your tips and tricks also works very well. That piece of information is something that will stimulate the brain to think about the action and put it into action easier.
Although giving tips is great, remember to let her discover most of the game. She needs to get used to the mechanics rather than copying what others do.
The last important thing is for both of you to have patience and get used to the world of games. Practice makes perfect.