The only thing she did wrong, that we know about, is trust him with their daughter. Divorcing him certainly wasn't the wrong thing to do.
I know plenty of women who'd say that if their loving hubby raped their little 14-year-old sister, he'd have gotten off pretty easy if all they did was divorce him and send him up the river for nine years. Down close to the Gulf Coast where I live, this guy would've long since been a scattered collection of fish turds.
She should never have let this monster anywhere near her daughter. It's not as if she didn't know his history and that leopards don't change their spots. Really stupid. Even if she'd said, okay, but only a supervised visit with me there, or several large men with baseball bats, it wouldn't have been sensible. (This monster probably followed the Groene killings on jailhouse tv.)
He was around 23 when they got married less than a week before the daughter's birth. I wonder if the mom was only 14 or 15 at the time herself, and that's why the wedding was delayed. Then they had another baby that same year... that's got to be tough on the mom. This "mom" of a 14 yr old may still be under 30, and still under the spell of the thug that raped her and got her pregnant when she was teenager. Her parents did not protect her or her little sister from him, and she failed to protect her daughter from him, too.
Of course this is just speculation...