Would they have to be charged or just questioned?
Likely if they won't testify by their own choice they would be subpeoned as a hostile witness by the prosecution and then probably plea the 5th to avoid testimony, correct?
According to the law library:
hostile witness
n. technically an "adverse witness" in a trial who is found by the judge to be hostile (adverse) to the position of the party whose attorney is questioning the witness, even though the attorney called the witness to testify on behalf of his/her client. When the attorney calling the witness finds that the answers are contrary to the legal position of his/her client or the witness becomes openly antagonistic, the attorney may request the judge to declare the witness to be "hostile" or "adverse." If the judge declares the witness to be hostile (i.e. adverse), the attorney may ask "leading" questions which suggest answers or are challenging to the testimony just as on cross examination of a witness who has testified for the opposition.
Pleading the fifth
In American criminal law, "taking the Fifth", also known as "pleading the Fifth" or "commanding the fifth", is the act of refusing to testify under oath in a court of law or any other tribunal (such as a Congressional committee) on the ground that the answers that would be given could be used as evidence against the witness to convict him or her of a criminal offense. Although similar to the right to remain silent when being questioned by law enforcement officers, and coming from the same source, namely the Fifth Amendment in the Bill of Rights, the right to refuse to answer when under oath has a longer history than Miranda rights.
In many jurisdictions other than the United States, witnesses may be compelled under threat of contempt to answer questions even when the answers would incriminate the witness in a criminal offense. (However, in Canada for example, Charter §13 provides automatic use immunity.) In such jurisdictions, such statements given under compulsion of subpoena cannot be used as evidence against that person (which is not the case in the United States). The American system has the benefit of not exposing the witness to the risk that law enforcement will be able to obtain collateral evidence based on the witness's testimony that law enforcement could not possibly have learned had the witness been allowed to remain silent.
A witness may not refuse to answer questions on the ground that the answers may reveal matters that may incriminate other persons (with the possible exception of married couples -- see Spousal privilege). Generally, a witness may not refuse to answer any relevant question put to him or her unless the answer would incriminate that witness. One common application of this rule is the practice of providing limited transactional immunity to a defendant as to specific crimes, thus narrowing the protection against self-incrimination, and permitting a court to compel answers about those crimes. The victim of a crime may be detained and compelled to answer questions, as a material witness, even if the victim does not want to "press charges," on the theory that the crime is of public concern, and the right does not extend to other parties