There is not a lot of authority on this question. It's more of mind teaser. But Prosser says that it is like the cases of substantial factor discussed on page 274-75. See Prosser's hornbook on page 266-67 for further reading.

If Driver had hit the horn button the injury still would have happened because Rusty disconnected the horn. But Driver has been negligent, and the accident is the type of risk that Driver created when she failed to hit the horn. Shouldn't the law require the negligent actor to compensate Ted?

But if so, why doesn't the law require negligent actors to compensate victims of accidents more generally? For example, suppose that Speedy Green is going 80 mph on a residential street. A block away Horace begins to cross the street and dies of a heart attack without ever knowing that the car was approaching. Should Speedy be liable for the wrongful death?
a) Yes.

b) No.