I feel like this has always been a difficult thing for me. When someone does something they shouldn't do, I don't know how to honestly tell them without me looking like the evil mother hen, or something along those lines. It also makes it worse if they are young and/or possibly a bit oversensitive. Then, if they have their little army that will defend any of their friend's stupidity, it makes things worse, and well, I end up looking like D-bag when I'm not trying to be.
However, it's hard to ignore when they're doing something wrong either. I don't think it's good to smile, nod, and let it slide, because the person may honestly not know what they are doing is wrong. That's usually what I assume. I give them the benefit of the doubt that they honestly just don't know. It still makes it difficult though, because I'm not sure how they will react to it once I tell them.
I'm the type of person who likes to get along with everyone the best I can. However, I like be honest about things too, and when I feel a person is acting irrationally, I want to do my best to help them, and thus I tell them open and honestly that they're making irrational decisions. As a friend, I would rather have someone be honest with me, than always agree with me and never say anything bad because it may hurt my feelings or piss me off. I feel that makes a person a poor friend in general, if they aren't willing to talk openly and honestly with someone they call a friend.
I'm not a perfect person either, heck, I'm far from it. I do irrational and stupid things at times. I make bad decisions at times also. I pick fights when I shouldn't. It happens to everyone. We all do and think things that we regret eventually. Life is a learning experience. We are all going to do stupid things, but we also have the chance to learn from the stupid things we do. I also don't think it wrong to ask for and accept, or heck, even listen to someone when they're trying to honestly help you. You can choose to accept it or reject it, but I think the effort is somewhat worth it. It shows that a person cares enough about you that they want to help you, and I don't think it makes a person a D-bag.
NOTE: This isn't geared toward anyone in particular, it's just an overall life experience rant of how I feel about helping people at times.