According to the CFE Prep - Investigations study guide:

''It is recommended that any witnesses who are considered potentially volatile be interviewed without advance notice. Surprise should be employed in any interview that is considered potentially volatile. In many instances, the potentially volatile respondent is unaware that he is going to be questioned, and will therefore be unprepared. If the interview is not conducted by surprise, the interviewer runs the risk of the respondent not showing up, showing up with a witness, or being present with counsel. A target's friends, relatives, and romantic interests often make for a difficult interview. They perceive that the fraud examiner is deliberately targeting someone close to them.''

Interpretation for this scenario:

Brianna is described as very protective of her boyfriend, making her a potentially volatile witness.

Best practice is to avoid giving her advance notice (which might increase hostility, resentment, or allow preparation).

Conducting the interview with little or no notice reduces the risk of interference, ensures she is unprepared, and prevents her from appearing with legal counsel or allies.