I've interviewed quite a few people in this age range. The ideas below are based on those experiences:
Has she watched any videos of interviews? It can often help to see what others do in an interview to get a sense of how long a response should be, the appropriate level of detail, formality, etc. After watching a few interview videos, she can move to pausing the videos after the interviewee responds and your DD giving her response (out loud). Then, after doing this a few times, with a different set of videos, your daughter responds and then watches how the person in the video responded.
It may also help to have a list of 5 ideas she's trying to insert somewhere into the interview. For instance, she might know that she can include the following: volunteer experience 1, volunteer experience 2, particular school project, particular school project 2, and her love for [something related to the job]. I think for this age group, it's hard to know what might be meaningful. Then, when questions are asked, she is trying to match it with something off this list rather than every life experience. Obviously, there may be a question that doesn't match at all, but it should help for many questions to simply have fewer stories/examples that she needs to be be able to share clearly and be excited about. If she's really nervous, the list can be on a sticky note next to her phone.
Similarly, she should know 5-8 words or phrases that she should be inserting into the interview. These should come directly from the job description.
Finally, if she's not placing her phone on a stand or securely in one spot, I'd suggest doing so.
Good luck to her!