Backing a friend up in public is loyal. Telling them they were wrong in private is more loyal.

Public defence costs you almost nothing and makes you look good. The harder version happens afterwards, with nobody watching, when you say the thing they do not want to hear and risk the friendship instead of your image. Both matter, and the order matters too. Standing beside someone first is what makes the honesty land as care rather than as piling on.

So do both halves. Hold the line in the room, then find them later and say it straight, once, without an audience. If you only ever do the public half you are a fan, and if you only do the private half you are a critic.