They were so inexperienced that they and their Major didn't tell her to GTFO out of their interview. The Major in PIU and command could have easily done so. They could make a case she was attempting to influence the PIU case by simply being a Major asking pointed questions about a PIU Sgt in a hostile tone. That kind of thing would tend to encourage officers stop talking to the investigators. She had no business being there. The better option is to delay the interview until she's out of the room.