So last week a buddy of mine was using nightmare and when his opponent won map choice/first player. The opponent chose his map and then when my buddy pointed out nightmares trait, the guy goes oh well I can still pick map and let you have first turn. I thought the person with map choice HAD to take first action. I thought the defer option only happened before map was chosen. Could I get some clarification on this pplease??
Thank you

You are exactly correct. Winner of Initiative Roll chooses First Player. First Player chooses map. You cannot choose map and go second.

From Rulebook P. 4 "All players reveal their completed forces. Then, each player rolls 2d6 (reroll ties). The player with the highest total result gets to choose who will be the first player. When playing with more than 2 players, the player with the next highest total gets to choose who will be the second player, and so on."

Rulebook P.5 "The first player chooses the map where the
game will occur. "