The EJ20G is the first incarnation of a turbo motor in Subaru's EJ motor line. It was first used in the '89-93 Legacy/Liberty. It was the top motor almost everywhere but in North America. It was then slightly revamped and used in the '93-'96 Impreza WRX/STI.
There is nothing "special" about them other than being the oldest, they are usually the cheapest. Some say they are the stoutest, but that is likely just because they were less high strung than later years.
EJ20Gs are easily identified because they have a cast 90 degree elbow on the inlet of the turbo (no pipe under the manifold).
You don't state what car your friend has, but if your friend has an '02+ WRX then he cannot use the EJ20G. Maybe the shortblock, but not the heads or intake manifold. There are sensor compatability issues.