There are various masks--if this one is bad, have them try ones that aren't so 'closed'. They make one that's a nasal pillow, the sleepweaver I told you about...some that are full face, and some that just cover the nose with a chin strap.
Be sure to ask about the ramp setting, so it goes up gradually--that worked better for me, at least. As long as your mouth isn't covered, hopefully you won't panic with it--but see about a way to have a bell or something attached so if you move your head and need it off, your CAL will hear?
You may find that laying with your head up a little bit will help, too.