If I buy an expansion, does my friend also need the expansion to play it together?

[Tempus14]

Both own the base game currently (i.e. this is not a remote-play-togehter setup).
Do both players need the expansion to play it together?
(This is not a question of morality, but of curiosity as I have experienced both systems with DLCs in the past.)

[AronFJenks]

Only one of you needs to have it. That person should be the one who starts the game.