A fin in the 58-62 cm range will probably help lower your planing threshold with your 8.5 m2 rig to the extent possible.
As soon as she has the basics, it would be good to spend some of your sailing session sailing with/alongside her.
It's the first basic skills sessions that really seem to disrupt relationships.
As soon as she feels she is "getting it" she'll be on her way. The Go with a smaller
fin should be OK for her if you (or the pro instructor) teach her to stand slightly off center and tip the upwind rail down a bit to stay upwind.
Hope this helps,