No matter how much you claim or genuinely think that two people are going to be ' best friends forever' or how much you 'love' or 'understand' each other, within a split second this can all change and over what?  Yup you guessed it over a guy that she hardly even knows as opposed to someone you have known for over a decade, someone whom  you grew up with. It really doesn't take much to strain a relationship does it? 
When you start seeing your friend fall for an arsehole, one of those lowest guys in the world who unfortunately is your friend as well,  do you step in and warn her or do you just let her watch and fall? Crossing that fine line? Sometimes when you have the best intentions at heart it can come back at you and bite you in the arse. It really amazes and saddens me at the same time.