Nothing like getting bitchslapped into a harsh reality by a couple plants eh? I can tell you that the mg is very easy to "misdose" likely meaning they were overferted or deprived most of the grow, both of which will definetly impact yield. Dude definetly needs a lot more info before we can help, from the info uve given it would be almost ignorant to suggest anything to help as it would be a shot in the dark, for all we know he needs new and better everything but maybe lighting. So, I'm assuming indoors but ill ask anyway, indoor or out?
What were the temps/rh with lights on/off?
Ph of water?
Soil?
Soil ph?
Vegged for 2 motnhs, how long were they flowered for?
What was the growth like? Short, bushy, nodes stacked like a deck of cards?
Tall lanky with 3+inched between each node?
A little of both?
Was any training incorporated?
How tall did they get in those 2 months of veg?
I hate to be a jerk but I've seen people pull more than that off 3 auto's and its obvious he knows somethings up as he is disappointed so hopefully he's willing and eager to take advice with an open mind and apply it. I know its a lot and there could still be a million variations and factors that caused it, perhaps the room set-up? Was his room "up to par"?
But if we can get even some of these answered we can likely help u a lot more. But at this point I'm gonna say, shotty genetics and perhaps inexperience(?) which ultimately leads to missing pieces (ie no oscilating fan, high temps, etc..) And look at it this way, at least he's got SOMETHING the he grew himself, not a lot of people can say such.