Can I remove a creature someone's sacrificing for an Emerge cost?
Unfortunately, you can't prevent him from saccing his Hooded Hydra.  What happens is this:


Friend declares he's going to cast Decimater
Friend determines the cost (in this case, Emerge)
Friend pays the cost, including sacrificing Hooded Hydra
Decimater goes on the stack
Decimater's cast trigger goes on the stack
Players now get priority, starting with your friend.