It happens to everyone. You get a call from your bezzie with news of a decision so terrible that you need to lie down in a dark room for a week. Sure, you can go off at your friend and tell them how stupid they’re being. But it sure as heck won’t stop them from doing the stupid thing.
Stay calm. Be honest, but consider your friend’s feelings. There’s a reason they decided this way, and with a little gentle direction, they might come to realise how blatantly ridiculous it is. Or they might not. At the end of the day, it’s their mistake to make.