My husband is the next of kin and we have been handling all of her financial/medical matters without having the POA or a conservatorship.
NOW is the time to get that. While it is unlikely care providers and those who need to be paid will balk at his non-legal handling of affairs as long as they get paid and he makes decisions they agree with he has no legal power to enforce anything until he gets guardianship and/or a POA.
Does my husband have any legal authority to make this decision having the "next of kin" status?
It is best if he goes to court and gets appointed her legal guardian. When it comes to decisions like the one he is making it is best if he has legal authority. Most medical providers have no issue with "next of kin" when it is for supportive medical decisions. When it comes to withdrawing care they prefer legal authority to prevent possible wrongful death suits from other family members.
The doctor however is balking at the choice we have made to remove her from support.
The family cannot force the medical provider to go along with the plan. Sadly this is exactly why it is highly recommended that everyone have an advance directive that spells out how much end of life care they want. If she doesn't have this and the physician/care team won't support it your husband may have to get another doctor who is supportive of end of life decisions once he has legal authority to make them. Best of luck to your family as this is NEVER an easy decision to make.
"That's just my opinion, then again I might be wrong." Dennis Miller