My friend asked me to make a vegan, gluten-free cake for her wedding in August. Any suggestions on approach more than welcome!

Find out if she will be happy with chocolate cake. I think they make the best vegan cakes. You might browse Gluten Free Girl blog to get tips on vegan cakes and see what ones she has made. You will need to find a gluten free flour mix that you like. Time to start experimenting. I was going to say to order Thomas Keller's Cup4Cup product, but it seems to have milk powder. Good luck, it sounds like a lot of trial and error ahead for you. Try Babycakes famous recipe (they have a book, too), see if it makes a cake.
http://www.epicurious.com/recipes/food/views/Vanilla-Cupcakes-353909

I found the cookbook called the intolerant gourmet very helpful for vegan and gluten free recipes

I can't help with the vegan and gluten-free part, but in terms of approach, I thought Smitten Kitchen's series on making a wedding cake was very informative. http://smittenkitchen.com/blog/category/wedding-cake/