I don't know, and can't imagine, what cruel things this friend has said about your DGC but I could see a situation when there is a bit of banter - and that can escalate from funny to cruel in seconds - where perhaps you said something slightly derogatory about one - maybe referred to scruffiness or tardiness - which your friend mistook for playfulness without recognising the signals.
you can say things but she has absolutely no right to
I am very suspicious of repartee and banter.
I know someone who thinks it's funny to poke fun at everyone he meets . Mostly he gets away with it but occasionally it gets taken the wrong way and it can be upsetting.
It's exhausting when she visits - AIBU or is it just me?
Couldn't organise anything - Boris needs to go
Tearing the family apart - freeloading brother