Hi S.,
It sounds like there is a lot going on here and that you are only hearing one side of the story. The relationship between the nanny and the mom is very strained and that is not healthy for the children. I think she needs to sit and make a list of the complaints she has about the nanny, and then review it to make sure she is not being petty. Did they always have this adversarial situation? Then she needs to sit down with her boyfriend and go through the list. Get his "input" on why "this nanny" is the one he and his parents seem to favor. Also this is a warning flag if his parents are involved in the decision of the nanny for her children??? are they paying for the nanny?