We have been cruising
DCL since Day 1. We were on the Maiden Voyage of the Disney Magic which was their first ship. I can assure you they sold booze since the very beginning. I can't remember if they allowed you to bring booze onboard since Day 1 but soon after they definitely did. They used to let you bring unlimited amounts onboard of any kind of alcohol but now they allow you to bring 2 bottles of wine or a six pack of beer per adult. All beverages (including if you want to bring a favorite soda) MUST be in your carry on bags. A wheeled duffle or suitcase that fits through a standard airport screener (all carry on bags are put through an xray screener) helps when bringing it onboard. Depending on what time you get onboard you may be carrying your stuff around until cabins are ready. Last week our cabins on the Magic were not open until 3 pm and we boarded at noon. We had swim stuff with us so spent the afternoon by the empty adult pool have a drink or two!
Note: any booze brought onboard can only be consumed in your own cabin. They can't be consumed in public. If you want to bring your bottle of wine to the dining room they will charge you a corking fee. The fee is not for the physical act of opening the bottle, but for bringing a non Disney purchased bottle to dinner. You will still be charged if you open the bottle yourself.
We just got back from our 47th DCL and I brought on a bottle of wine and have brough something onboard for most of those cruises.
As far as booze being sold onboard....they have a drink of the day which comes out to $6.13 with auto gratuity. They have beer specials and sometimes two for one drinks by the pool and in some lounges. Several happy hours were listed in the cruise activities as well. They have a full selection of wines and spirits and will make just about any drink you want.
Tell your friend you WON the debate!
MJ
With no casino, they have to make money somewhere. 
