Your friend maybe stopped the evolution with the following methods -
- When evolving, press B for the evolution stops, making Dratini not evolving.
- Maybe he leave Dratini in the Daycare, since Daycare does not evolve any Pokemon.
- Maybe using Everstone. (When this item is held by a Pokémon, it will be prevented from evolving by Level Up means).
- XY Game - Maybe get the Dratini in the wild with high level, since in Black 2 and White 2, you can find Dratinis from 40-70 level.
If this is XY, the most common from level 40 Dratini , it's because in XY at Route 21 using Good Rod, you can find Dratini at Level 40!
If you level it up to Level 41 in a battle or with a Rare Candy (not with the Day Care), without it holfing an Everstone, it will evolve into Dragonair and then at Level 55 into Dragonite.
Also you're correct when you say it should evolve at Level 30.
Source for Dratini at Route 21.
Hope this helps!