First, you're going to want to set up your partner as a new Mailer. After inputting their name and email, under the Settings section, you'll place a green checkmark next to the Uses Internal Opt-Out link line. This will allow them to upload their Opt-Out List to yours. In the Mailers Access section, turn on Override System Settings and check List Cleansing only. This will just allow your Mailer to use our system in order to scrub their list if they happen to find that more convenient. Under the Add Mailer to Campaigns section, use Don't add Mailer to any Campaigns, then hit Create Mailer.
Now, scroll over the Campaigns tab and click View Campaigns. Scroll over the Campaign name in question and click Edit from the options that pop-up. Scroll down to Choose Mailers and Give Access to Selected Mailers will need to be enabled. Now just add a check mark next to this new Mailer's name. Be sure the Email New Access Keys option just below this checkbox field is green checked, and then Update/Save settings. This will send the Access Key to the Mailer and give him the access level that you requested. The Mailer will want to use the Upload your Suppression List option in their portal.