In D&D it is usually stated when a spell can burn objects, but those objects need to be flammable. You can't just burn stone. And think about it, evokers have the ability Sculpt Spell. It allows them to use AOE damaging spells on the enemy without hitting their friends, a very cool ability indeed. But, right now, even if the fireball didn't hit your fighter, he is still going to be stranded in the middle of a lake of fire.
