Being a supportive friend doesn’t mean always agreeing with your friend. It means supporting that his mind reached a certain conclusion — or decision — and you respect him for it. You support a friend by respecting his mind, his reasoning, and his judgment. If you didn’t respect these things you wouldn’t be his friend, right? If you do respect his mind, and you happen not to share a particular conclusion — you can still be his friend. This assumes you think his error in reasoning was an honest and understandable one, although not one you would make.