One of the hallmarks of constructive criticism is that it is presented in such a way that the recipient understands the criticism is about their behavior, that it isn’t personal. However, any group of people brought together by mutual concerns are going to develop personal history. Some things will be personal.
If you’re delivering criticism to someone with whom you have a history, you can’t pretend that history doesn’t exist or that it doesn’t affect your ongoing interactions. The same is true if you’re delivering criticism to someone who disagrees with your friends. And again when you’re criticizing someone who disagrees with you on an issue that elicits an emotional reaction in you.
None of us want to think that criticism is actually about our behavior. It’s much, much easier to dismiss it as the product of someone else’s biased thinking. It’s much easier to say, “This isn’t about what I do, because no matter what I do, this person is not going to like it or me.”
Does this mean you can’t criticize someone constructively under these conditions? No, but it does mean you have a bigger task ahead of you. You need to acknowledge your history and your biases before it occurs to someone else to ask why you haven’t, and you have to honestly and non-trivially analyze how that history and bias affects your position. With all that out of the way, your point at least stands a chance of being heard for what it is.
This isn’t easy, and it isn’t comfortable, but transparency is one of the requirements of effective, and thus constructive, criticism.