You can reinstate the affiliate via Seller Admin > View/Edit Affiliates, this pops up a new window where all of your affiliates are listed, and there's an undelete function up at the top.
You will need their affiliate ID, which they can provide you with from their own account. It's displayed on their Affiliate Admin screen. Just enter that number into the Undelete field on the View/Edit Affiliates page and they will automatically be added back to your program.
