Well, as Kaltia pointed out you're in the wrong forum
Anyhow, IMHO that's a bit late to dual class her! It will take ages for her to get her thieving abilities back and she'll be so weak compared to the rest of your party as she goes through the first few mage levels! I'm not sure if you can still manage to get her high up enough to get back your thieving skills, but even if you do, it's a waste of time I think. Most people dual-class her much earlier. Still, if you're decided on it, I wish you good luck!