Hi, I'm sorry you are having to deal with this.it sounds like all liquids are running down into her Lungs . Ask the nurse for a Yankeur suction . This is similar to what the dentist uses when he needs to keep the mouth dry while he is doing work in the mouth and to prevent swallowing of secretions.
I would use this while your Mum is cleaning her mouth. I would also use oral sponges on a stick. Don't let her fill her mouth with water. She should be able to get a good clean with the sponges and the suction will suck any moisture away to stop any aspiration from occurring. I'm pretty gob smacked that the Nurses have not offered this.
Thinking of you,
Tammy

Caregiver/advocate to Husband Kris age 59@ diagnosis
DX Dec '10 SCC BOT T4aN2bM0 HPV+ve.Cisplatin x3 35 IMRT.
PET 6/11 clear.
R) level 2-4 neck dissection 8/1/11 to remove residual node - necrotic with NED
Feb '12 Ca back.. 3/8/12 total glossectomy/laryngectomy/bilat neck dissection/partial pharyngectomy etc. clear margins. All nodes negative for disease. PEG in.
March 2017 - 5 years disease free. Woohoo!