Well I tried that once, on the basis of 'treat others like you want to be treated', so I listed the mistakes she made in her job interview at the end so that she could improve on those points in her next interview. I wasn't being an asshole about it, I did it very carefully, it was a very hard thing to do because I'm usually very nice to everybody, it's second nature to me to not ruffle any feathers, I really framed everything in objective terms, made sure to tell her why I did it etc.

Next thing you know I'm sitting there in the conference room with a crying girl, and I'm not talking a silent tear crying, I'm talking full-out wailing in a way that the secretary from another company 2 doors down (!) came to check up on us, to see what was going on. So I come out like a bumbling idiot trying to make up an excuse for what was going on to that secretary, trying to calm down the girl I was interviewing, and she probably still went back home telling everybody what a heartless bastard I was.

So now even when people ask I'm very careful to phrase any feedback in the most general terms. It sucks for those who could handle the honest feedback, and I hope that others that I will interview with in the future are braver than I am, but this 'giving honest feedback' hasn't worked out for me.