Centerpieces, color scheme, jewelery for the bridesmaids, even the cake. Perhaps you could have the wedding at an arboretum or some other flower related place? That could be gorgeous. I got married at a local park with a mansion on the premises and it was very reasonable price-wise - and it was a county park so no alcahol was allowed which worked for us. - google "sunflower themed wedding" etc. and you'll find TONS of stuff - and click on "images" to see pictures of things (some are awful LOL, but some are BEAUTIFUL)
There are some great books out there - some about budget planning, etc. Honestly, weddings can get REALLY freaking expensive, so if they want to put some of the money toward something like a house or whatever, I suggest enlisting friends and family. If she's a little bit outside the box instead of the traditional "princess" wedding girl, it could be a lot of fun to figure out what you all can do.
See who has natural gifts/talents, etc and ask them for their expertise or whatever as a gift. I had a friend who was a great seamstress and she sewed all my bridesmaids dresses. Another friend got the flowers (I did daisies for my bridesmaids, roses for me). The park was naturally beautiful, and we decorated the home with Ivy and flowers from the gardens of friends. I made the centerpieces. I loved how much fun we had.
Just remind her to ENJOY herself. I did, but not every bride does if they get wrapped up in how the wedding looks to others. If she can build a day she'll enjoy, it'll be wonderful :)