I'm not so sure. I've been on CPAP for over 5 years, and recently, I have been noticing that my lungs are feeling like I have been breathing in moisture from a cool sauna. It doesn't go away. There is nothing that has shown up in any tests, that would cause such a thing, so I do wonder if the lungs CAN get too much moisture from CPAP. I use it for nigh time sleep as well as naps, so I get lots of use. So far, I can find no other explanation.
I don't think it is possible to breathe in *excess* moist air via the machine. It only fills up so far. The key issue is how often is the humidifier cleaned per the instruction manual, using the correct solution. Warm moist heat can help breed bacteria. The tech who set up the system should have instructed your wife on all the cleaning procedures. You might want to try posting this same question on the sleep forum. Hope this helps!