I think the first time he presented it almost as a joke. I've tried to ask myself what I would say if a man in my life with an ex threw out a remark like "why don't you get rid of her for me? You're a nurse, you can just inject her. She's crazy and I'm afraid she's going to hurt my baby".
If she didn't cut off all communications after the first time, he had his answer. She might not help in the murder, but he knew he could count on her not to betray him and he knew he could persuade/threaten her to help with the coverup.
At this point, I'd like to think (giving her the benefit of doubt) that she was trying to distance herself from him. I wouldn't be surprised if her comment about PF being 'evil' was made after he threatened her kids. When he called and told her it was over, but he wanted her help to dispose of the phone and send texts, she may have decided that nothing would bring KB back but maybe if she did what he asked, he would leave her (and her children) alone in the future.