I sure hope not, unless you are an EMT or something. In that case, it would sort of be a job requirement. Ideally, I wouldn't want first aid from someone who doesn't want to help me. Then again, that's why they call it "first aid" - take what you can get as soon as you can get it. It may be loosely related, but have you seen the last episode of Seinfeld? In my opinion, it shouldn't be illegal, but it is immoral to refuse to help a fellow human with a basic, urgent, immediate need.
An employer cannot force you to be a first responder against your will. The only time you can be forced to is when it involves a crime you are somehow related to or something that compromises the nation security of which the authorities will step in. Contact a lawyer for more details.