At least in California, Good Samaritan laws will protect someone if and only if they are rendering medical aid that is in line with their training. Meaning if you know CPR and attempt to resuscitate someone who has no pulse and they die / have brain failure -> the law will protect you. If you know CPR and try to remove a bullet from someone's brain and they end up in a vegetative states (IANAL but) -> law probably won't protect you.