Actually, I think you should give your honest opinion, but failing is the best learning tool. I had a business partner a few years ago (also a friend) that I wish would have failed a few times before we partnered.

The issue was that I had a few failures under my belt and knew what not to do..but I had to constantly battle against his ideas that were very likely to fail, but he just didn't know because he lacked experience. It eventually led to the shutdown of our business (which never really got off the ground).