Slitherine

I want to play a map from the '42 DLC against a friend but he does not have this DLC. Is there a way to play it?

Yes, only one person needs to have a map for both players to be able to play it.

Follow the instructions here, host the scenario on the PBEM server, and your friend should be able to play.
viewtopic.php?t=28384

Just keep in mind that most DLC scenarios were NOT specifically designed for multiplayer. So while they are playable, they may not be optimized or balanced.

I altered a map from the DLC so it is fit for PBEM. Maybe I need to see the settings of the map again? My pal gets this message "cannot start game because required content package is missing"

What a minute... You say the DLC scenario's are playable BUT may not be optimized? So this means that once I alter a scenario it can not be played?

No, they are still playable after you altered it, just not necessarily balanced for MP.

So what does that messagemean he recieve's?

AFAIK, It means, it was altered with an older version (1.04?) of the scernario editor and played with in v1.05