How do I tell the boss she’s too hard on a great co-worker?

Question: “How do I tell my supervisor, the CEO of small association, that she’s being overly critical and it’s hurting morale? Example: Our annual conference is an exhausting week-long event. Our days start with pre-breakfast set up checks and continue through dinners with association members, VIP speakers and  sponsors.  Last year when we returned, our boss called a “post-conference critique” meeting to point out all the things that went wrong! I felt so sorry for the conference director. The conference went well, but the CEO was so negative.  I privately told the conference director that I thought she’d done a great job, but she left her job soon after (for a better one). What can I say to our boss—who’s been in her position for 20 years and who does have many good qualities—to be less critical? No wonder she keeps losing conference directors.”  –Wish I’d spoken up