[FireBeyond]

Good Samaritan laws cover a layperson attempting to do good (so long as you're not doing something a reasonable person would realize to be actively harmful).

In most states, "ignoring" a DNR order is covered similarly. Point of interest:

Even as an EMS provider, with a patient who has a valid and active POLST (Physician's Orders on Life Sustaining Treatment - a more detailed DNR, that offers advice on comfort measures, intubation, and the like) - if the family wants heroic measures (CPR, etc), then we are required to do so (though my personal moral and ethical compass has issues with this).