How To Deal with Someone Who Thinks They Know It All But Really Doesn't in 3 Easy Steps
I lead a team of 12 analysts at work, they get a lot of things wrong a lot of times.
I've come up with a formula when dealing with mistakes and it's fairly simple: all you have to do is to ask the right questions, you don't have to tell them anything. That's right, you can make them realize their own mistakes and have them come up with their solution (otherwise, it would be you doing their job instead).
I'll tailor fit the answer to someone who "thinks they are right."
What you need to prepare:
The Conversation in 3 Easy Steps:
1. Create a good atmosphere
Ask him how he is. If you work with him, you would know a bit about what they are up to. Ask him about his quest to become a better designer, this will help you transition to a discussion on his project
2. Ask him how he feels about his project
2.a Pull up his design. Start the discussion with the Good Parts "Speaking of design.. I wanted to get your opinion on your project" Let him start with the good parts, and agree with him on the good parts that you actually think are good as lying won't help. If he says "everything is good" let him elaborate what makes it good. Having him write these down will be very helpful.
Remember: If at some point you don't agree with what he says, do not say he is wrong, it'll only make him defensive. Say 'okay' and just tell him to write those down. Later you can compare what he thinks is good and what good really means for your project (2.b.i).
2.b. Ask him about what he thinks needs improvement
Ask him what about things he thinks he can improve on. Having him write these down too will be very helpful.
Remember: Have him say these, avoid the urge to enumerate what needs to be done properly.
2.b.i He REALLY Thinks he's Right
Here's where your Company's vision/goal becomes useful. If somewhere there states for example: "Making UI's Simple and Elegant" but his work is the complete opposite, then you can go back to 2.b. and ask him "How can we make this design more simple and Elegant?"
A model design also helps, show him a design that fits the company/department/project's vision. Ask him what makes the model design look/feel great, have him list these down. Make him do a comparison, his notes should tell him everything. He might have listed down "I have 15 useful buttons on my navbar" as part of things he did good, but finds that it is contrary to a "Simple" UI design. Then you can go back to 2.b. and ask him "How can we make this design look/feel great?"
2.c. Now he's acknowledged his work isn't perfect and might have provided his own suggestion (some might even be really good this time), but now is time to provide your inputs too
Avoid telling him what to do, suggest instead with phrases like:
3. Let him know you're on his side and you're there to help him
Because you are. If he does a crappy job, your doing a crappy job too.
In fact, I encourage you to see this in another perspective, one that is trying to help him. Because if you're genuinely trying to help him, it'll show and chances are he'll be more receptive. The day will end with you doing the world what we all need, a bit of kindness.