I think it's fair game to call ideas stupid, and that's what he did. It's his assessment, your opinion may be different, but in this case I'd personally say his

I think it's fair game to call ideas stupid, and that's what he did. It's his assessment, your opinion may be different, but in this case I'd personally say his judgement was correct. As far as feedback on ideas goes, this is a harsh criticism, but it hasn't really crossed the line into abrasiveness because it's not a personal attack. It's just a given that even smart people can and do have stupid ideas. At no point did he say or imply anything about the person raising the idea.