Mum struggling with mucus before radiotherapy

Hi, I'm looking after my mum who is 80 and has very aggressive throat cancer, she had a 5 day round of chemo 4 weeks ago and is due to start radiotherapy in 10 days. She can not swallow anything now, only tiny sips of water and is fitted with a feeding tube. Her mucus is really bad, she is struggling to breath, can't lie down, can't sleep and is constantly coughing and retching and having panic attacks due to the breathing problems, she says it feels like she is drowning,  horrendous to watch and bot be able to help  there has been no help from the oncology team with this mucus problem  my mum says there is no way she is gonna be able to lie flat with the mask on for the radiotherapy and just wants to give up. How have other people managed with this? Any advice on how to ease this problem and how to cope with lying down for the treatment. She is so messed up and her mental health (and mine) is on the floor!