I had to give a deposition once in a work benefits type case. I was tape recorded as my boss and I were asked various questions related to the case. As someone else said, you are sworn to tell the truth, so in some cases, that means that it is better to answer honestly to all the questions. Generally, if you don't have specific factual knowledge of something, it is better and acceptable to answer with statements like I don't know...not sure...or etc. On the flip side, if you do have relevant info(good or bad) state what you know.
All in all, it is not that bad a process to go through, but hopefully your friend will be ok.