Did the other adult have to buy the refreshment package? That's normally a waiver requirement.
The GS drinks can be limiting. Are you sure big drinker wouldn't want more?
The big drinker in a GS without a DX would have to visit deck 17 for every drink 11am to 11pm and that only includes cheap beer and wine. The mixed cocktail hours are 5pm to 8pm but for a big drink it might not be appealing to wait until 5pm for a mixed drink, especially having to go to deck 17 for every drink. That's no fun.
If you put the under 21 in the OV with the big drinker then big drinker can buy the DX drink package and the under 21 doesn't have to buy anything.
Normally when requesting a waiver for just one person buying the DX they require other people in the same cabin who are over 21 to buy the refreshment package. While not as much, it's still several hundred for a 7 night cruise each.
Leverage the under 21 in the OV to get around that by putting them in the OV together.