Why doesn't my friend drop her toxic friend?

I have a bestie let's call her L. L has a friend D who used to be my best friend. We also have a mutual friend, O so there were 4 of us in this friend group. D and I were always closer to each other we used to tell we were each others soulmates. But after she got her first bf she started to act really weird. A lot of things happened but long story short both me and O dropped her bc she did many things that a friend would never do. We haven't talked in well over a year but I'm still friends with O and L. Now L for some reason still hangs out with her but keeps complaining about the things she did (that she used to do to us). Now turns out D was talking with L's ex who was harassing L bc he wanted to get back with her. Said ex sent all of their texts to D who just casually told this to L but didn't want show their conversation so I'm sure she said some pretty mean things about her. So she told us all this and then obv we asked why didn't she just stood up and left? She said she didn't have the heart to do that. And now we want to go on a vacation with a big friend group and L wants to invite D for whatever reason. How could I convience her to just block her and never talk to her again? I'm getting desperate bc me and O don't want to be in one place with her for multiple days. I'm straight up sick of the thought. Pls help