Can next of kin override a living will?

Highly hypothetical. Say a person, John Doe has a DNR and living will stating that he does not want any type of life support used. John and his daughter Jane are alone when he begins having a heart attack and states that he changed his mind. He does not want to die and wants all measures taken. He then loses consciousness before the ambulance gets there and goes into cardiac arrest. Therefore, noone but Jane heard him say he wants to live. Will the paramedics and hospital listen to Jane and try to save him or abide by the living will and let him die? What if he never said he wanted to change it but Jane (as his next of kin, uming she's an only child) states that she wants to disregard the living will and try to save him anyways? I know it's morally wrong and all and I'm not saying I'd do it I'm just doing some research