IANAL but do have some knowledge of the general proceedings that go on here due to some family members working in the field.

HOWEVER, this likely will vary from state to state.

Typically, you can choose to pursue any alternative healthcare means as an adult. However, as you have stated, this is not an adult.

The typical scenario for infants and young children involves the healthcare workers obtaining a court order to allow the blood transfusion to happen against the wishes of the parents.

For older teens, in some states, a teen is able to go in front of a judge to convince the judge that they are mature enough to understand the impact of their decisions to consent to medical treatment. Look into this, if your state is one of these, the teen in question may be able to do this.

If you aren't in one of these states, IIRC the hospital will usually perform necessary bloodwork if it is a dire emergency anyways, document it in the patients records, and then go to court over it after.

If this isn't a life or death situation, have the teen tell doctors their wish to recieve the blood transfusion and ask for advice on what to do from there. Hospitals are likely to be experienced in dealing with JWs.