Urban mythology strikes again.

The "good samaritan" doctrine would most likely shield the volunteer provider from liability. Even if that doctrine didn't apply, the volunteer's conduct would be judged by the standards of their own profession for other similarly-qualified providers acting under the same (emergency) circumstances with the same limitations as to available medications, equipment, etc. And their malpractice insurance almost assuredly would (1) retain and pay for their defense attorney, and (2) indemnify them against losses in the extremely unlikely event of an adverse verdict.

Unless there was clear evidence of seriously negligent error on the part of the volunteer, accompanied by clear proof that the negligence was the cause of a serious injury or death that wouldn't have happened otherwise, no competent lawyer would be stupid enough to file such a case in the first place. Plaintiffs' lawyers get paid only if and when they win. They don't like to work for free.