Ex-Wife continually threatens unfounded lawsuits – what gives?

My fiancés ex-wife keeps threatening lawsuits and to take full custody of their 10 year old daughter (currently they share 50/50). She keeps saying that my fiancé is a bad father, bad role model and that he breaks the law via noncompliance with their separation agreement. None of this is true. She literally makes up stories about him, and I honestly think she has started to believe them. She does not have any legal grounds for any of her threats and has been “fired” by 2 lawyers so far – they told her they could not help her with what she wanted to do because she has no grounds etc. Yet, she still will call up my fiancé or send him e-mails/texts, threatening him to high heaven. It’s getting to the point where it’s almost abusive! My fiancé does need to talk and communicate with her on occasion because of their daughter; otherwise he would just not answer the phone/respond to e-mails etc. She left him for someone who subsequently left her. Why is she acting this way? Why is she so bend on making our lives miserable? Have any of you been in a similar situation? How did you handle it?