The most important thing about bad work isn’t whose fault it is that the work is bad. It’s that it is bad work. The most important thing to do is to fix that bad work.
You may find that, while the solution being proposed isn’t a good fit, the problem addressed is important. Ask the person what he is trying to achieve with his design idea. Is he thinking of a different user group or business need? Keep asking “what?” and “why?” (maybe in different ways) until you understand what that problem is.