I understand that it’s really disappointing after so much prodding to have a student fail so miserably on an important project. But sometimes we have to fail to learn what doesn’t work and to hopefully increase the motivation to try harder. Depressing as it may be, you have to grade honestly - It’s not fair to the other students to be more lenient on some nor is it ultimately fair to this student to keep them from dealing with the consequences of a very poor job… You could offer to the student and any others in the same boat the opportunity to do make up or extra credit work. But to be honest, it seems improbable that it would do much good other than to show you care… One final thing, I’m no teacher but as an engineer we often review each other’s work. It’s important to separate “I would have done it differently” from “this just won’t work” or “just plain wrong”. People do approach things differently and often just because it’s not how I would have chosen to do it, doesn’t make it wrong. I don’t really think that’s quite what you are dealing with but it’s important to keep in mind. I once had a high school physics teacher who was so incompetent at her subject that unless you parroted back exactly the same phrasing and words from the textbook on a question she couldn’t understand that it was possible to mean the same thing with different words. She was so awful that if you had a question in class it was pointless to ask her - we had to ask nearby classmates. Fortunately she was fired but not before wasting the opportunity to teach.

I'm amazed that people like that make it to junior year in college. Even senior year I had people in group projects who were totally unengaged and either did terrible work or none at all. I often wonder if this issue is solved by more prestigious schools or if they also have people who probably shouldn't be there.