Beyond sending your constituent a link to your signup form or using the "manage my preferences" link at the bottom of a previous mailing, there are two other ways you could approach this.
In this instance, what you can do is go into your own signup form and fill out the opted out address. This will then send her a confirm opt-in email which she simply needs to click on the link inside that mailing.
Alternately, if you have an email FROM THE OPTED OUT ADDRESS specifically saying that they would like to opt back in, you can forward those on to us here at firstname.lastname@example.org . Make sure to include some information to help us identify your account if it's not obvious. We'll get that taken care of and let you know.
I hope that helps, please don't hesitate to let me know if you have any further questions here :)