Nobody likes to listen someone saying their work sucks, but any sane person would welcome mentoring and ways of avoiding unnecessary work.
One school of teaching even says that you should not point out mistakes, but focus what is done right. For instance, instead of pointing out incomprehensible code as bad, you should point out where their code is particularly easy to read. In the first case you are priming others to think and act like crappy programmers. In the later case you are priming for thinking like a skilled professional.
