A friend of mine is obsessed with her fiancés ex, a perfectly harmless woman to which her fiancé has a DS(9). She uses every opportunity to be quite vile about her and more often than not (I and one other close friend) are her sounding board when she wants to have a moan. This is most days and she'll go a step further send photos of the woman ripping holes into how she looks or what she's wearing EG "look at how fat she looks" 
The lady hasn't done anything wrong, she's settled down with a partner and newborn and doesn't cause any problems for friend and her fiancé. Access to fiancés DS is amicable and he and friend have him every weekend and one night in the week, friend is close to fiancés DS and the mum has no problem with friend being a big part of her DS life. Friend is goady on social media about how she does so much for the child, then privately messages me and others in our friendship group saying how he'd be so much better off living with them full time.
Friend seems to be insecure in her relationship as she and fiancé have struggled to conceive, she goes above and beyond to paint a "happy families" picture on the internet then will ask me and other friends to comment smushy remarks on their pictures so "fiancés ex will see it" despite there being no evidence to suggest she's ever been jealous.
The pinnacle for me came yesterday when after another rant about her, friend said she hopes she dies whilst on holiday. I don't recognize what my friend has become and have never seen her act so cruel toward anyone else, I find her constant slagging off very draining and am struggling to just roll my eyes like I usually would. I don't like confrontation or wanted to get involved so haven't stuck up for the woman until now but have now sent a message saying she needs to grow up and stop being so cruel.