Hey there,
Since SCS released their MP feature, they have also implemented the feature of players gaining the ability to see DLC's that they do not own.
This feature is also implemented in TMP. So if you own "X" DLC containing tuning pack/trailer and your friend does not own the same "X" DLC, your friend will still be able to see the content of that DLC containg tuning pack/trailer.
This feature does not apply to map DLC's however