The original game was Sins of a Solar Empire.  The first expansion pack was Entrenchment which added extra content to the original game.  The second expansion pack was Diplomacy, which added on even more extras. 

If you play Diplomacy, you are playing the full game with all the extra goodies.  If you play Entrenchment you have all the goodies minus the ones in Diplomacy.  The original game has all the content the game originally shipped with, none of the expansion packs.

You should play Diplomacy, since it has all the content.

Didn't the new patch deal with that?

I think it might be better to start out with the original, plain Sins.  It's a little less complex in that he would only have two tech trees to worry about and not the Entrenchment and Diplomacy tech trees.  That way he could become comfortable with the basics and then move on to Entrenchment once he knows the races' tech trees.  From there he could move onto Diplomacy.  Why start with all of the complexity of the four tech trees when you really don't need to?