You obviously need to charge her. She really can't fault you for asking her the cost of materials. I mean, you're contributing labor for nothing at all, you can't expect to pay for it out of pocket as well. Here's what I would do:
Start thinking about the design, and spend a little time making an estimate about how much you think it will cost. If she agrees to that cost, then go ahead with the project. If it turns out to be a over, you can bite the difference. If it is under, you can give her the difference. This way, you're not paying this huge amount out of pocket, but at the same time, you're not asking her to pay whatever cost it turns out to be. Just make sure you do a pretty good estimate. Think about EVERYTHING, right down to the thread.
-------
Before you slip into unconsciousness, I'd like to have another kiss,
another flashing chance at bliss, another kiss, another kiss...
The days are bright, and filled with pain, enclose me in your gentle rain.