Hey DG,
yeh, there's actually lots you can do beforehand, i'm teaching my wife atm and have learnt a lot from it.
firstly, there's a podcast, 'in between swims' get her to listen to the one released on the 30/10/09 called 'The kayaking mindset' by Heather Herbeck, it helped my wife a lot, (its worth it for the guys too) and has to do with the mental aspect of kayaking, which is what my wife has had to work hard on. Secondly, a boat she feels comfortable in is very good, my wife hated the river runner but loved the creeker she tried, felt safe and it worked well with how she paddled. she hasn't learnt to roll yet, but she's not worried about that, since she knows how to wet exit and has practiced it so the fear of being tipped over has diminished. Heather Herbeck is also part of a group called 'river angels' (google them for their web page) and has learnt a lot reading what they've posted, found inspiration in what they do, and has see that there are girls out there doing great stuff, and has since set her standards higher for what she thinks she can achieve herself.
Lastly, dont loose your temper, if you loose control of yourself due to frustration, you've lost her from the river, if you need to talk, raft up and talk quietly in her ear rather than talking loudly from a boat's distance away, she'll feel safer and won't think your angry with her (personal experience)

It definitely sounds like you need to inspire her with a lot of confidence though, and you're the best person to do it! so take heart, good luck, and do your best!