Speaking of lightweight spokes, I've had great luck building wheels with DT Aerolites on a couple of wheelsets. I'd have to check the weight, but they are indeed light, the lightest offered by DT I believe. But they're expensive, relatively speaking.......like with most things bicycle-related, the main question is "are you willing to spend $x to save y weight. That's true even with the original example of DT straight gauge vs DT 2.0/1.8/2.0. 62 grams of weight savings on a wheelset costs $x more. And in that example, there is the issue of the double butted spoke having another advantage as well.

The statement by the friend in the OP, "double butted spokes save a lot of weight" is probably a little misinformed and over the top, but I have to admit I've done some pretty expensive things on bikes over the years to save 60 grams here, 60 grams there.