In the fogwash its actually an overheat sensor, not a gas sensor,.. and its the only model i've ever seen fitted with one on the exhaust. Disabling it isn't dangerous in this instance, but other boilers may be different, so its understandable that your mate takes this stance.
that's a big NO NO I asked my pal to do it he is a boiler engineer he refused me point blank said that's asking for trouble if gas builds up etc you have had it and any body around be warned.