Is there a way to tag photos in the iPhoto library with Faces on iOS?
Something like a sequence of screens that just asks for confirmation like "Is this John, yes/no"?
I'm afraid not. You can do places and whatnot from your i-device, but sadly no faces ability, although I have the feeling Apple will do it in the future. You can quite easily do it on iPhoto however. If you need any help in getting started at all in iPhoto, here's Apple's own support page: http://docs.info.apple.com/article.html?path=iPhoto/9.0/en/pht2297cf5f.html
Hope this helps.
This might do it - untested but found whilst Googling for another Ask Different question
PhotoTime - Automatic Face Sorting & Keywords Tagging for Your Moments https://itunes.apple.com/us/app/phototime-automatic-face-sorting/id846435251?mt=8