I think it's fair enough to ask a parent, relative or friend to give a little bit of notice before visiting, if only to check that it's convenient - and I would definitely do so if it had been specifically requested.
My own son and his partner, and my daughter are fairly laid back about such things and wouldn't expect me to visit "by appointment only" but I would normally ring to check whether they're, for instance, planning to go out, are in the middle of eating/cooking, or are especially busy.
However, I think your friend's daughter's attitude towards second hand clothes seems on the face of it most rude and unpleasant. I was very happy when my friends gave me clothes for my children which their children had grown out of. In the past I did occasionally buy charity shop clothes for myself and my children - still do for myself. I mostly buy new clothes for our grandchildren but I have given charity shop clothes (and toys/books) to my grandchildren when I have found something of particularly good quality and in good condition - I launder/thoroughly clean them first. They are always appreciated and I have seen the children wearing the clothes/playing with the toys.
Is there a possibility that your friend "popped in" a little too often and - when she did visit - didn't pick up "cues" that indicated it was time to leave? Is it also possible that she gave second hand clothes that were in poor condition? As someone else said, perhaps her daughter has had experience as a child of having to wear noticeably well-worn and unattractive clothes. A little diplomacy and kindness on her daughter's part wouldn't go amiss though.