42 plants is a very, very vague term (which is why you always see number of plants and not floor area or lighting watts). A growing method called Sea Of Green has many plants crammed into a small space, being flowered (necessary to produce the desired buds) before they get very big at all. If I had to guess I'd say he was growing 1 plant per square foot in a total area 6 feet by 7 feet. That is... a couple of small garden sheds (one in the growing phase, the other in flowering) with a bit of room to move in each... exactly as reported.
At absolute best that would have produced 3kg of bud which would have produced 300g of extracted resin oil. That's about a year or so supply for heavy medical use (cancer treatment) or 2 to 3 years for an arthritis patient. Cannabis is an effective painkiller (THC and CBD) and an effective immuno-suppressive (CBD), the latter of which is incredibly useful in treating arthritis... and the pain killing properties are obviously useful.
From a non-user.