You could let her know you will be using her email as proof of non payment and that you will be consulting with a collection agency regarding her refusal to pay. You could send a registered letter to this effect. Sometimes people just need a little reminder that they need to uphold the terms of their signed agreements. Just a thought
I agree... this is what I'd do. Make it look like you've consulted credit bureau assistance, or legal assistance. Might scare them into paying.