The fruit of the Anaqua (Ehretia anacua) tree, also known as Sandpaper tree is indeed edible. The only recipe that I find is in the book, Edible and Useful Plants of Texas and the Southwest by Delena Tull. This is a recipe for Anacua Jam. Due to copyright restrictions, I cannot reproduce the recipe here. However you should be able to find the book in your library or any book store and get the recipe.