Doesn't living for others assume you know others' true needs?

Some people say you should live for others. However, doesn’t living for others inherently assume that you know others’ true needs, ie what’s best for them?

Take as an example: suppose your friend is head over heels in love with someone you are pretty sure is not right for them. Would living for others mean (1) convincing your friend that her lover is not a good fit (maybe objectively the best for them but you are in some sense imposing your will on others and also better be right as what if you are wrong and the lover is actually a kind person) or (2) cheering your friend on whatever they may pursue (though you think their path is incorrect)?

Some might say just be a good listener but sometimes ppl want your actual advice or actions and merely listening isn’t enough.

Your thoughts?