Unit test are not just for people who write bugs (parent comment), a good start would be to test for the 9th row the problem was asking for. Since you can't test all possible values a representative sample would be good enough.
Just to be pedantic: It's not TDD if he already wrote the functional code ;)
I beg to differ. There is always design work, you made design choices and even if they were careless choices you always have a final work that might be great or just poorly designed.
Practice helps if you can learn from the good and bad moves on both sides of the board. The article mentions analysis, have you tried that? Great players draw from a big pool of previous experiences and not only their own.
Just to be pedantic: It's not TDD if he already wrote the functional code ;)