Your interviewers next week will be different from your coach buddy. Your buddy is there to help answers questions you have regarding your preparation, and give you one case practice + feedback, prior to your first interview. He/she will not be part of the official interview process, - you can use them to ask questions and practice.
Treat that buddy carefully though, and make sure you practice your case session with them only when you feel at least 80% ready for the actual interviews, so you can make the best use of the session with them.
Hope that helps ! Let us know if you have more questions as you prep for the big day :)