EXP is granted based on being near a kill, not being the one to kill it. Each monster/minion/champion grants EXP that is divided equally to all those who are in that area.
If he leashed jungle he could have stayed long and taken EXP when the jungler killed the camp. To be level 2 though probably means he took EXP from more than one camp however.
You didn't say who the other team's jungler was but what could have happened is this. Mundo helps their jungle steal your blue (Lee generally starts red and they took advantage of that) then he helped leash the enemy red after. Those camps should be enough to get both Mundo and the jungler to level 2