Speaking as a person who deals ALOT with proper lubrication and corrosion prevention etc I can tell you that the use of WD-40 of breathing apparratus or any rubber parts is just plain silly.
WD-40 is a very poor performer as a specific lubricant/penetrating oil etc. It is a general purpose water displacing lubricant. Its biggest failure is the fact that it evapourates thus loosing its ability to protect metal. Moisture is then free to do its dirty work on the bare metal. As it is a solvent it will also dilute other lubricantes and allow them to be carried off leaving the metal exposed. Its use on rubber etc is just plain silly. There are many other proper lubricants to use.
Depending on what the Husband and Wife team were wanting when they presented thier regulators to the instructor, they should get the idiot to pay for a servicing.
I do a big search for information, talk to manufacturers, service techs etc before using anything on or for a particular product. There are proper o-ring lubricants and silicon sprays/greases for rubbers and textiles and for breathing gear depending on the required use and environment.
Your regs freeze up so spray a bit of WD

buy the right reg for the conditions!
It is really bad to hear that a supposed "professional" would be so stupid as to inflict possible harm on someone requireing support.:shakehead
Just because someone gets away with it or personally uses it does not make it the "best solution" or a safe solution. Like spraying "quick start" into a diesel to get it running is a last resort not a solution to a poorly maintained/cold engine. Like the guy smoking a cigarette working on a gas engine?? "Never had a problem before" Ugh.
WD-40 on breathing equipment hahahahahahahahahaha
