I believe that collaboration is saying what you need to hear, not what you want to hear. Now, the thing is doing it in a way that reflects respect.

Some people are not prepared to receive feedback, and will probably react strongly against it. It's easier to believe someone else is wrong than to accept that you are wrong. This becomes reinforced as more people support a wrong idea.

It's also useful to sticks to facts, not opinions, and emphasize the problem, not the person.