I'm so sorry you are all going through this. I'd ask your mother to give permission to your doctor to let him speak to you about her condition in detail. With greater understanding of why he won't treat the infection, you can then advise her better. A second opinion is probably wise. Good luck to you during his difficult time.
It could be what her doctor has decided for her or there could be another possible reason why treatment is not being carried out. Try talking to your mum about it, and if possible her doctor, so that you can have all your questions answered.