Can you play Saint's Row 3 DLC co-op when only one of us bought the DLC?

I got a season pass for Saint's Row 3, but my friend doesn't. Can we play the DLC together?

No you cannot. It (The season pass info page) says that each copy of the DLC requires the base game in order to play.

To the other side of the question if your friend has the base game, but no DLC, the answer is still no because he will not have the content contained in the Season Pass.

Source

Yes, you can. I just picked up SR3 on the steam Thanksgiving sale, and went through Gangstas in Space with a friend that only had the base game.

At the same time each paid for DLC was released, they also released free Viewer Packs which allow people without the dlc to play through the dlc missions in co-op.