I went on Craigslist and found fabric for really cheap. I am sure a thrift store would have scraps as well. When my friends come over to quilt, I pull out all my scraps and the craigslist fabric and let them use them to their heart's content. This way, I am not invested in the fabric (which I am in most of my stash).
I don't let my friends down in my spare bedroom to pick fabric from my stash because I'd just have to say NO too many times and I would probably come off looking like a greedy witch who didn't want to share, even though all the fabric is mine, purchased with my money.
To me, it is like someone seeing your pantry and expecting that you give them food because you have more than they do... but you don't get to tell them not to take all the "good" stuff. Of course, they are going to gravitate toward the yummy food, just like quilters gravitate toward the beautiful, expensive fabrics!
I agree that separating your stash and pulling out what she can use is a good solution. You can tell her that you have projects planned for most of your stash and don't want to have to replace fabric when you do the projects because they were used in other projects.