Hi O,
With sand bars and sandy bottom's like this one at honeymoon island in the aitutaki lagoon you can teach her yourself. There is no kite shop or school in the cook islands. You can hire windsurfing gear from locals on raro but as for lessons, i'm pretty sure there is nothing.
Def make an effort to get over to aitutaki as its 1o times better then raro and has no reefy bottom. Have fun! One foot and honeymoon islands are wicked for teaching on, waist deep sandy bottoms. Have a read on some other posts about aitutaki and you will see there is info on places to stay and who to hook up with for boat support etc..
XX angie