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

I think you’ve got the meaning of that phrase wrong. Living for others means that you are living a life that is shaped by other people’s validation. You are not being yourself and are trying to please others because you want them to approve of you – that’s what the phrase means.

@ak I’m referring to altruism.

al·tru·ism: the belief in or practice of disinterested and selfless concern for the well-being of others.

@weeveeship It wouldn’t have been ambiguous, had you mentioned altruism in the question. Anyway, I really think you still haven’t got it. Altruism is concern of well being of everyone who you personally don’t know. Volunteers working overtime in hospitals helping pandemic victims are altruistic. When I help my friend, I’m not altruistic, I have personal interest in his/her well being. I’m just being human. Nothing altruistic or selfless about it.
Let me answer your example directly. If you are as close to your friend, as you are saying, you won’t hesitate to voice your concerns with concrete proof (or evidence) to back your claims. It is in your interest to save your friend from a potential heart break. If you don’t have proof and are just basing your judgment of the relationships on your ‘feelings’, you probably need a little bit of introspection to discover why you feel that way.

Seems to be a reasonable assumption

”............know others’ true needs?”

Maybe not know another’s needs but realizing that they have a blind spot or vulnerable.

I have a senior friend who is presently in friendship with this women that has a nasty side to

her ( others had been subjected to her passive aggressive tactics when she doesn’t get her way).

I had tea with her the other day and second time I warned her and she stated that she knows.

So she is duly warned and takes the consequences and thus my part in it is complete.

I don’t take “living for others” to mean knowing what’s best for your friends. I take the expression to exemplify someone like Mother Teresa, Albert Schweitzer or Cesar Chavez who worked to improve the lives of others. Some needs are pretty clearly identifiable.

We are not very different to each other.
There are lots of ways we can know that someone else needs help or could use a hand. Assuming you know others needs is not a problem in that context, those needs are the same as your own. Water, food, shelter, safety, and maybe a good internet connection.
If “living for others” means service to others, sure that makes the assumption that you know others’ needs, and that is true.

Yes, it’s possible for people project their imagination’s idea of needs onto others, but that is not altruistic, that’s a selfish position. That’s just someone being bossy and overbearing.