Tammi, definitely contact your client - be honest and tell her you'd love to have her feedback, not only for your own growth as an editor, but to help boost your new business...
She's likely just busy and doesn't realize how very important it is to you.
If it was me I would just write a quick, friendly email noting how nice it was to work together and how much you appreciate her offer of a testimonial - you're looking forward to receiving her feedback and working together in the future on similar projects (something to that effect)