Let them know to have a word with the ground staff at check in as well, they can notify the crew via the purser so they can keep an eye out during boarding and have the crew working that area help settle them in and check on her during the flight.
I found with nervous flyers it was often helpful to know what exactly about flying was troubling them (for some it's the enclosed space, others heights, or the not being in control, or turbulence) once you know the more specific issue you can try to minimise your daughter''s apprehension.
For example if she didn't like the feeling of a small space, opt for seats further back in a larger cabin section as she would have a wider view of a larger area than if she were sitting at a bulkhead staring at the wall.
If it's turbulence or weather that scares her, knowing a bit more about the kinds of turbulence and why they happen can help too. Often it's the unknown factors that frighten people and once they know a bit more they feel less worried. She has the perfect person to give her the basics, at least
And of course there's always distraction. If I had time I used to take the nervous flyer for a walk around, show them the galley and how we do the service, or talk to them about where they are going and what they'll do when they're there. Takes the focus off the now and reminds them of why they're going.
Anyway you probably know all this but maybe it'll be useful. The crews are large and you'll always find someone who is good at helping. Smooth travels to your daughter, I hope she enjoys the wedding!
Edit: just realised it's daughter's friend, sorry.