You'll have to have that person text in from their own phone to be added again. We can't allow someone to opt someone back in, it's all up to that individual person.
Long code opt-in
On a long code, simply have your contact text the word START to your number to be added back.
On a shortcode, you'll need to have them text a keyword in your account. If you don't have a keyword, you can add a free one under TEXT MESSAGING > MOBILE KEYWORDS.