You can use either of the fins, but once you get a larger sail (=>7.0 m2) you should find that the 48 cm Race fin gets you planing earlier and will help you to stay upwind when you are fully planing and riding "on the fin".
Your girlfriend will appreciate the smaller Drake Shallow Water 41 cm fin as it will allow her to learn to beachstart more easily and should provide all the fin she needs with smaller rigs.
She is going to have to learn to "rail" the board very quickly to stay upwind.
If she wants to sail upwind, she only needs to stand slightly off the centerline on the upwind side to "tip" the board slightly upwind rail down to allow the shape in the bottom of the board to take her upwind.
If you look under the nose of the board (or maybe on the deck near the vent plug fitting) you will find a serial number.
Send me that number (use the Private mail feature if you prefer) and I can break the number down and tell you what year and month your board was made.
Hope this helps,