Is a vet an option? I'm gonna be direct... If she is so sick that you think tubing will kill her then she is very close to death now. She's gone from 8 pounds to 4 pounds? Most people say that you you tube when they lose 5% of their body weight, so we're past that. 
Sadly, it might be too late, once they get this thin they are very hard to save. If she were mine I would tube fluids ASAP.

Sadly, it might be too late, once they get this thin they are very hard to save. If she were mine I would tube fluids ASAP.