what do you do when you see two people you both know and care about start a relationship that you know is going to end badly?

I don’t say anything because when I was in one I hated it when people kept telling me it would end badly which it did but I kept falling out with people who kept saying it. Alot of the time people know what someone is like but their feelings rule what they already know. That did was like that for me , I knew there was something not right but I was determined to give her a chance just in case I was wrong or she had a good reason for it. I just support them through it and at the end of it too.