Your copywriter needs input. Provide your copywriter with your current marketing material, and customer contact information. Encourage your staff and suppliers to talk to your copywriter. The more information your copywriter has to work with, the better the copy will be.
This brings up the issue of trust. Your copywriter will guard your confidential information. If you have concerns about this, ask her to sign a Non-Disclosure Agreement (NDA).
Here's a Wikipedia article on what a NDA should cover.