Can I second or third the condom rule? Some STDs are completely invisible and don't scream, "Hey! Look at me! I'm going to kill you!" HPV can lay dormant for twenty years, and is being linked to all sorts of nifty cancers. Maybe not as frightening as AIDS, but take it from someone who had a large portion of her cervix removed, it's still scary.
Don't worry about the fact that he gets away with things you don't. You don't get away with them because she cares about you and he's a f*ckbuddy. In my relationships, part of the attraction between my husband, Easy, and our girlfriend, Asha, is that they like to be rough. Early on I tried to keep up and play as rough, but I found it frightening to see him in that role, and it made me feel tense around him in our real life. I depend on Easy to be caring husband and loving father and caretaker when I'm sick or scared--I couldn't handle seeing him as a scary person. She felt "trashed" when you treated her rough because she counts on you to show her respect--I'm guessing that with him it's all about sex and with you it's about feelings. She doesn't care how he sees her, she doesn't see him as a big part of her life.
I worry about this complete tossing-aside of responsibility. You guys have a kid and it kind of scares me that she's involved with someone who is on cocaine.