Is that love or stupidity?

I know that I have no right to interfere with someone when it is personal. But can I just ignore someone crying inside our room? I know that I am insensitive sometimes but knowing the reason why that someone is crying is somewhat a different story. I know that it's her personal life, or let me make it more specific, it is about her love life. Maybe I don't understand why she keeps on believing that everything will be going just fine. Based on my knowledge about what happened, I can say that it was not okay. I'm sorry to say but I really don't like someone who is unfaithful. I don't know what exactly happened. All I know is that I don't like what I heard about what he did. Maybe I am just angry because someone who is a part of my life got hurt because of him.
I asked her if she still loves him and she answered me "Is that a question?". So I presumed that she still loves that guy. She also told me that it's not easy to let go of someone who is special even if that someone caused so much pain to her. Is that what you call love? Or is it what you call a mere stupidity? Will you stay for someone who does not deserve you? Will you stick to someone who just found someone else to fling with?
Maybe I just don't understand people who keep on hoping for another chance because I haven't experienced it but I think everything has a limitation. I don't think people will destroy their lives because of their lovers/ex. There are other relevant things to think about.