I have, but don't encourage it. I know what I prefer to sew with, and what works the best for the items I make. An example is a customer wanted to make baby sheets out of a heavy fabric that would normally be used for drapes. She really didn't know the difference, until I explained it to her. Also, sometimes the pattern design on a fabric won't work for the item you are working on. It might be too large, too small, or not line up right.
Before you agree to it, make sure the fabric is something you can work with, and if you use the fabric the end product can be the same quality that you usually produce.
One third piece of advice, if you do use their fabric, make sure you have extra in case the first item tried doesn't work out the way you want it to or takes more than you figured as you might not be able to get more of it.