So sorry your daughter is experiencing this! I have suffered with UTI's all my adult life and have had to take way too many Antibiotics , so I know just how you both feel. :(
It sounds like you are doing the right thing with the d-mannose, but in my experience it really only works *before* you get a UTI, to prevent it.
I would also really, really recommend the book "You Don't Have to Live with Cystitis" by Larrian Gillespie, MD. This book probably helped me more than anything else. You can order it on Amazon.com I believe.