This might be a good time to do a pair programming exercise. Go through the code with them so they can see your process. For example, review the requirements,

This might be a good time to do a pair programming exercise. Go through the code with them so they can see your process. For example, review the requirements, write test cases to cover those reqs, then refactor the code to work or even start from scratch if it's not that much code. The best way for a developer to get better is to be shown a better way to do things.