I have not served on many PhD committees, because my department does not offer them. I am on several committees in related disciplines, but few of them have gotten this far in the process (a few students dropped out before this stage) and the other one that is further along than this went well. I would have been okay with passing him/her on my part of the exam, but a few other committee members felt the s/he did not even work to learn the follow-up material for theirs (and I agree).
I feel just terrible about the whole thing and keep wondering what went wrong- should the advisor have held back on the oral exam when the written exams were not stellar? (I think so!) Should this re-take have been postponed to allow more time for the student to prepare? It's not clear. We did not have the student come back in afterwards, but I saw him/her with her/his advisor afterward and gave her/him a hug (I'm the only woman on the committee, by the way). I told her/him to come and see me soon. I want to encourage this student to apply to another PhD program (maybe in my field instead), because s/he is smart and I think s/he could do it. But today is just NOT his/her day.