Hopefully I won't become a sacrifice for my leader/manager in the future

Well, people management is never easy because everyone is unique and requires different kind of management. What I told my friend was to observe and learn more about their characteristics and their goals. Also, as a leader, he needs to be fair, flexible, open-minded and never discriminates. For example, one of his team members (who is a very good programmer) is a loner and hardly joins the team for lunch or outing. The other team members find it difficult to communicate and work with him. But does that mean my friend should penalize him? And does that mean this guy is not valuable for the team?

No, this guy should not be penalized and he is still a valuable member of the team due to his skills. The thing my friend should do is to manage him differently and give him his own space to work in. His existence might ruin the team spirit but there are still things which he can do – maybe those assignments which only need one resource/person.

But that is just 1 type of people that a team lead might need to handle. Honestly speaking, there are many other more problematic type of people out there. Some will only talk without doing work. Some will act smart in front of you. Some will talk bad about other team members. So on and so forth.

Again, different people different kind of handling. And that is why managing people is an art in itself. Last but not least, remember never to discriminate people for being different.

Hope it helps.