Ok - first off there are several carcharhinid sharks that known as "blacktips". But at present I'm not aware of any species that only gets to about 30" long. It's not to say that requiem sharks can't grow that small - after all there is a couple of Sharpnose species (Rhizoprionodon) that are known max out at about 2.5-3 ft.
As for Blacktips- these are the presently known species.
Blacktip Reef Shark (Carcharhinus melanopterus) - is the species most often known to SW Aquarists- both public and Private. This is often the species that most aquarists think of when they say Blacktips. It reachs about 6 ft in length, and is native to the Indo-Pacific region, specifically to rocky and coral reefs.
Blacktip Shark (Carcharhinus limbatus) - is the species sometimes called the "Atlantic Balcktip" although it's a global species of Blacktip shark - found in the Atlantic, Pacific & Indian Oceans. This species is known to be an aggressive, and high strung species that usually averages better than 6.5-7 ft in length. However it's very capable of growing to 8-9 ft length.
Australian Blacktip Shark (Carcharhinus tilstoni) - this species is native to the Indo-pacific region near northern Australia. It grows to about size that's roughly about the same size as the Blacktip Reef.
Also the Spinner Shark is sometimes mistaken a Blacktip Shark (C. limbatus). But the point is all known "blacktip" sharks tend to grow to about 5-9 ft in length.
The only reasonable explanations that I can think of your friend stating that his working with blacks that stay under 30" - is that maybe your friend has found either a smaller sub-population of a known blacktip or discovered a new species of Blacktip. The other possibility is that your friend is actually just working with Juvenile Blacktips of one of the known species.