I'm a fan of providing concrete feedback to candidates after an interview, if for no other reason than because it was something I would want if interviewing myself.
However, I have someone that I'm not sure how to proceed with. I recently had a technical phone interview with this person, and it went poorly. There were some general questions that the candidate answered very well, but a lot of simple topics that the candidate had difficulty answering coherently. In addition, they showed all the signs of using google: a pause before answering questions, followed by a long and detailed answer given very rapidly, and then no ability to answer followup questions. When it came time to look at and write actual code, the candidate did especially poorly, flopping every question I gave them. The position itself is for a senior level developer, and I've run enough candidates through my questions now that I know that they are reasonable questions to expect someone with a few years of experience to be able to answer.
I'm obviously not going to continue the interview process with the candidate, but here is the part I am stuck at: is there any feedback I can give this person that will be professional, courteous, and potentially constructive? I'm pretty sure it isn't reasonable to mention my suspicions about their use of google to answer my questions. I would like the feedback to be constructive if possible, but I'm having a hard time coming up with something other than a polite variant on "You did really badly", especially since nervousness is often a factor in these things.
Reading some other related questions I understand that the first answer for many will probably be "don't give feedback". In this case that might be what I have to do. However, I still have a strong preference for providing some concrete and constructive feedback. I understand that, despite my best efforts, said feedback might not be well received. Given all of this, does anyone have any suggestions on how to provide concrete and constructive feedback to someone who bombed an interview fairly badly?