Should I bother to reply to this friend?

As harsh as it sounds, people like this are a drain on your energy. She is possibly depressed and you shouldn't let her bring you down with her. If you really value the friendship, then be honest and tell her that you're feeling a bit vulnerable right now and that you need to stay positive. If you don't think she'll respond well to that, then perhaps don't answer and let the friendship wither naturally.