It adds to his Steam linked ESO account if you log in to ESO's website with the same ESO account credentials and purchase it from there. You can't buy it on Steam since Steam doesn't allow it as he already owns the base game included in the collection.
All content in the collection will be added to his ESO account, not Steam so it won't appear in his library, but it's there when he logs into the game.
Sylvermynx wrote: »You might have to contact steam to get it sorted out. I don't use steam myself, so I really don't know how they're set up for contact, but there should be a knowledge base or help section somewhere.