Your scrawny friend had what's referred to as neuromuscular strength and it has nothing to do with aerobic conditioning, and yes, it can be trained.
Your friend also probably had lots of what's called "fast twitch" muscle fiber. The proportion is somewhat determined by genetics, but can be influenced by training.
There are many, many measures of human strength, power, and endurance. There's three energy systems, as well - neuromuscular (1-15 sec efforts), anerobic (few minutes) and aerobic (10min and up.)