Wider means more stable and thus for reassuring, important for confidence.
Narrower means faster while paddling, important to get early in the waves especially if not a strong paddler. Note that boards with a lot of curve in the outline (very wide at the handle but narrow of nose & tail) will paddle very slowly and with a lot of row.
I guess she should test some to get an idea of what could be the best compromise. Note that a "wide all around" shape (e.g a mini simmons) will feel much stabler than a pointed nose & tail design of the same width.
100 to 110 should be a nice volume for her.
This is the ideal kind of board for girls less than 70kg (beginner/intermediate) or up to 90kg (good level):
www.gong-galaxy.com/magazine/pics/1-hour-4-boards/- semi pointed nose for ease of balance
- wide for stability
- parallel outline on a good length, and a fast rocker, for paddling speed and ease of takeoff
- pulled in tail and thin rails to turn without too much rear leg pressure
- and of course... pink :-)