What he didn't mention, is that the human breath that you breathe out, also contains some formaldehyde and thus the reading on the device includes some formaldehyde from his own breath as well, which means, it should be a little lower than 35ppb for the e cigarette alone, like 34 or 33.
"Although there are analytical challenges in accurately determining formaldehyde concentrations in human breath (Moser et al., 2005; Kushch et al., 2008), the levels detected using a chemical-specific methodology fall into the low-ppb (parts-per-billion) range (i.e., <0.5-1.7 ppb) (Riess et al., 2010)"