There should never be a film on the water, if there is that also is getting into the lungs.
Have you checked the two filters on the machine? Are they new or clean? The recommended change interval is a bit agressive, but if you don't know how old they are might be best to get new ones and change them.
Are you running a HEPA filter in the room with your mother?
One member recently posted about waxy buildup inside the hose and water tank that turned out to be gypsum and loose drywall dust getting into his machine and causing the waxy buildup.
Presuming you have eliminated all external sources and the filters are clean the skin on the water is one of the tell-tale signs of the recall foam coming apart - your mothers machine is one affected unless it was procured after being "fixed". If it is under recall it means the same particles are in the air your mother is breathing. The link below is all about it.
https://www.usa.philips.com/healthcare/ ... src-update
If it has been more than 5 years or if your unit is recalled you should get a resmed version of the machine from your mother's DME. Also realize you can change your mother's DME at will if you aren't getting good answers from them.
In resmed's line of machines you will need an 'Aircurve' model, they come in the version 10 and the version 11 of the machine. Version 11 is most recent and was just released so may not be available yet. Version 10 has a long reliable track record and it good for 22500 on the motor and then it can b refurbed with a new motor for another go around.
The Aircurve 10 ASV is what I use, I suspect your mother needs the S or ST model. The Aircurve series has never been in as much demand as the airsense series so the odds of finding a genuine used machine are pretty low and you are better off with new.
There are people that sell what they say are ST or ASV or all types - they are selling the hardware from the low end machines and they have opened it up and flashed the firmware to enable all modes. Some people have used the Airbreak hack because they couldn't afford the premium price for the high end models. Nothing wrong with it if you don't mind the illegal aspect of it. Just know what it is you are buying.
For any Resmed model you get you should verify the 'run hours' on the machine before signing for it. On the series 10 you just turn it on and press the round button then scroll to the bottom and see the 'about' and it will list it. On the series 11, power up, hold a finger on both menu options for 20 seconds or so, then go to about and see the runtime.
They run them at the factory for up to 20 hours max, anything longer than that is a machine that was issued to someone else and then returned.
My wife's DME tried to issue her a machine with a bit over a thousand hours on it until I complained, then they magically came up with a brand new one that had 1 hour on it.