My friend plays a stupid amount of prevent all damage instants. If I attack, and use something with split second, can he still use his prevent all damage? Or with split second ending the stack, is he unable to do anything except block?
Note the text of split second:
Split second (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)
Split second only prevents casting spells and activating abilities while the split-second spell itself is on the stack. This means no player may take an action in between casting the spells and resolution. After that spell resolves, however, you'll still pass priority around normally (meaning your friend now gets to play other spells or abilities) before going on to the next part of the turn.
Split second only prevents casting spells and activating abilities while the split-second spell itself is on the stack.
If you were to cast a split-second spell after blockers are declared, he wouldn't be able to respond to it with his instant. But as soon as your split-second spell resolves, both players are again given the chance to cast a spell. He could cast his then.