Why are some people completely amoral?

There's an interesting letter up at Since You Asked. A woman is concerned for her friend who repeatedly gets very drunk and has unprotected sex with people, and doesn't tell her boyfriend.

Am I the only one who thinks that you have to be completely amoral to not tell the boyfriend? Even if she didn't have a boyfriend, wouldn't it be wise to tell the woman, frankly: If you want to party a lot and have wild sex, that's 100% cool. But every time this happens, you leave us at the bar, end up somewhere, and then call in the middle of the night for help. The next day you blame us. If you have a problem, we're enabling you. If you don't, you need to find a better way to party. Either way, if you ever need us to help you positively, we will always love you and be there for you.

Thoughts?