CO - it should not be an issue. She is capturing hot air from her heating vents. If she had CO issues, everyone in the house would have symptoms.
From the hot water heater, if that is gas, then the vent on top is venting the CO2 from the gas flame. Most CO poisoning comes from fuels that do not / have not burned well, like a cars exhaust. Our furnaces are vented to capture unwanted gasses, like CO2. CO2 machines attache propane tanks to a burner and the exhaust is circulated into a grow room to increase the CO2 levels. The thing is, if she is capturing CO2, she would have to make sure at lights off time she allows O2 to re-enter the grow space. Co2 during the day, O2 at night