If you love your friend and you see him/her doing something that could hurt them, it would be a sin not to warn them.

I sometimes find this very difficult.  Sometimes you can be misunderstood and if the person isn't ready to hear what you have to say then while in denial....they sometimes end up resenting you for it.  Of course if great friends, this is resolved later but at the time, its really tough.

This isn't difficult at all.  In fact, what you may consider to be a problem in their lives is perfectly fine with them.  One girlfriend was going out with this old, ugly guy, and she told me herself the relationship wasn't right.  They argued constantly.  I asked her questions about it instead of making statements.  It helped her think on her own and it didn't damage our friendship.

I'm not very good at it (although I mean well) so maybe that's why I have no fear confronting the situation. If a friend needs help, it is never difficult to do so.

a good friend of mine has a child that is abusive to her and i hate it but all you can do is try to be there for her cuz no matter what he does she defends him like a mother tiger.

This can be hard because you may be afraid that they will get offended and leave you. But if you do it in the way the Bible instructs you to do, that is to persuade them gently and with love, not considering yourself to be superior, then you will have a good chance of persuading them.

This one was the hardest for me, and I learned my lesson at the cost of my ex-boyfriend's life. He killed himself last year.