Losing my bestfriend

I've been friends with my best friend for almost 7 yrs, and we were like sisters. I went to my first house parties with her, double dated with her, and even attend all of her family events with her. She was the first female I ever referred to as my sister. She was always the confident, positive, and headstrong girl who could make friends with the meanest person on earth if she wanted to. Things between us started changing once she met her baby daddy. She started hanging out with the wrong crowd. Her personality started changing. After she found out she was pregnant from him, he started isolating her from her friends and family. He can't keep a job, and has anger issues. He left her with the first two kids..moved to another state and all...and as soon as he's ready to be a dad and bf again he'll call her with some sad story and she"ll run to his rescue. Now three years later they are going on baby #3 (I just found out she was pregnant on Mother's Day again) we argued about it b/c I feel like she's lowering her standards for this piece of shit. He doesnt even want me coming over to thier house, and i was in the picture before he was! Every time we do hang out, mall, etc, she always has to rush bk home to him b/c he starts bitching like a little girl! The funny thing is that he doesnt even help her pay the bills, but yet she runs bk to him like he's the man of the house! She is very smart with numbers, and with every job she's had..she's always promoted to a manager or assistant manager within the first two months. But she's no longer the friend she once was. I feel like our relationship is slowing fading and I don't have many female friends or ones I can call "sister" like I could with her. Every time I talk to her about it she gets defensive and acts like I'm just making her behavior patterns up. My husband thinks that I should let her go and struggle with her own mistakes. My mom thinks I should stop discussing her lifestyle with her for the sake of our friendship. But its so hard. She's my best friend. I want what's best for her and those kids and I can't just stand by and keep watching this low life destroy her character, control her, and keep impregnating her then kicking her to the curb. Idk what to do??