A lot! ;) And keep in mind that if they provide supplies, you need to be able to veto if they don't provide appropriate supplies. So give them a list and description, or go shopping with them. I'm making a T-shirt quilt for a friend, and we laid out her shirts ahead of time to calculate how much interfacing and how much sashing/border fabric we'd need (as well as to take pictures to keep track of where we'd decided to put each shirt). Then we went to the store together so I could help her with picking out the sashing/border fabric and show her possibilities for backing fabric (waiting to get that until we have the quilt top done - also gives her time to save $). Since we went to JAF, I was also able to give her a sale coupon to use.
And don't forget thread!