Eddie, we've been working together for ten years now and I'm quite happy with your "direct" approach because it's always constructive.
If you tell me a guitar part isn't right, them I'm happy to do it again until it IS right. The same with the mixes I've done for you.
At the same time, you're never offended if I ask you to re-sing a note that was flat (really?), so it works both ways. This is possibly
why we still talk most weeks. That and soccer.

If you need to sack someone there's no nice way, so just do it. If you lose a friend, then they probably weren't worth your friendship.
If the band is going to be professional, then all the players need to be professional - end of story.