My first post apparently went awry, so here it is again:
Hope I'm not toooo late! I am also fairly new to cake decorating, just finished my Wilton classes last month. I think you can cover the wire with fondant, for the branches, and I have also heard of dipping wires in chocolate, layer upon layer, so they would be safe to use with cakes. Shape them how you want them first! Then you can always wrap them, a real pain. Light bulb!! I just got an idea. If they are not eating the branch, you might try coating a 'real branch' with the chocolate lol!! Would save a lot of fuss.
The leaves just look like circles that have been pulled out of shape a little. And pinch at the base where they are to be attached to the branch, and a pinch at the top for the point, a more life like effect. Also brush a little red, &/or yellow on the green ones, here & there, as needed for effect, they are changing color after all, for fall. Let them dry well in odd positions, simulating curling leaves, etc... Vary in size... kind of like what we used to call the 'Dollar' plant.
These can be 'glued', with tylose glue, or glue of choice, directly onto the branch. These can also be made of rice paper, but you did say she had requested fondant ones. This is becoming fun, I just might try one myself! Good luck!