You leaving the game is worse than double jungling just saying. If people left games just because someone was taking cs, there would be a ton of supports leaving games every game. I get that jungle doesnt share exp from camps anymore, but think about it like this. If you double jungle, you help the other jungler clear faster so it's not all wasted. And when you two gank, it's more deadly because if you both have cc, it's a terror. If you believe this is a bad strat, just go tend a lane that needs tending. You need to learn how to adapt to situations. Is what he did good? No. Does leaving the game help or do anything better for the situation? No.
What would you have done differently if it was your first time playing league or you had no idea how to play league? You would have looked and realized he is taking the jungle camps, so you would go to another lane or camp and take farm. You wouldn't be giving up and afking. You would try your best to win. That's what matters more, not leaving because things arent going as you initially planned.
I am in no way saying it was not troll or toxic what the other guy did nor am I saying it is either. I am just saying you need to adapt to situation. Do what you can in game to help win