I would like that too. After a surgery a few years I sometimes have post nasal drip which causes bad breath. I have no way to tell when this happens. I would love a sensor that could detect it.
According to your sibling, there are already off-the-shelf sensors that detect some malodors. Maybe we just need to source already-existing components and convince an EE/manufacturing engineer to do a kickstarter?
The problem is that you need to know what chemical you are looking for that's causing the smell. Are there any sensors that can detect a wide range of smells?
A quick google search reveals a number of label free odor sensors, everything is early stage though.
Label free may not be the best phrase for what you meant though. Label free means you are looking at all biomarkers present in a sample, the primary challenge with label free is that we don't know every biomarker to look for or what every biomarker we see means.
The real problem is replicating our olfactory system. It can detect millions of compounds (even undiscovered and new) and we still have competing theories on how it works. One involves quantum mechanics[1], which is very hard to replicate with our current level of manufacturing.
I would pay at least $150 for that as a hard-of-smelling person. I would have far paid more back when I was single and lonely.