I disagree with this too.
I think its reasonable for a child to decline to be a source of support for a person who stresses her out and involves her in issues that she doesn't feel ready or able to deal with. I can see how that sucks from the other side, but it's an appropriate and reasonable choice for a person to make.
Staying friends would also be an appropriate and reasonable choice, if that's what the OP's DD wanted to do, but if she doesn't want to, she shouldn't be encouraged to invest energy in the relationship in the name of compassion.
I'm an adult, with a ton of resources, and even I have limits. I've let friendships drop because the people involved were too needy and the relationship required more than I had available to give.