People, even close friends, are bound to disagree from time to time. When that happens, do you support your friend, even if you think they are wrong?
Even people who agree on most subjects will disagree once and awhile, often about very important topics. It is bound to happen, because each person has a different background with different experiences and therefore different opinions.
No doubt something like this has happened to you at some point. For instance, you might not like a significant other they have chosen. Do you keep your opinions to yourself, or risk alienating your friend if they choose the relationship over you? What if it is serious enough to involve marriage, and you think it is a bad match? Do you speak up?
For another example, people often disagree about money. Perhaps your friend wants to buy a house that you think they can’t afford. Do you offer your friend support, possibly even financial support? Or would you be honest, with the potential consequence of offending your friend?
You can probably think of other examples. Maybe your friend wants to leave a stable job to pursue a dream job that carries the risk of failure. Or the reverse: your friend values stability when you think it is best to take a risk for something they believe in.
The key issue is a disagreement over something important. Do you think a good friend is someone who is cheerleader despite their own misgivings? Or do you think you are serving your friend better if you are honest about your own opinion, even if it might put stress on your relationship?
Related questions: What makes a good friend? What makes a friendship? When is a lie justified? What is truth?