Great question! I agree that asking directly or even hinting poses the risk of inadvertently hurting people's feelings, no matter how close or friendly the relationship is at the moment. The risk for hurt feelings -- I think -- has to do with people's ideas about tolerance to different food flavors and aromas. The risk is higher if comments/requests are interpreted as culturally-related. For instance, if I don't like the smell of kimchi or natto, is that necessarily interpreted as me having something against Korean or Japanese culture? (It shouldn't, but some people will wonder.)
Sometimes the risk is the interpretation may be based erroneously on personal-hygiene (as in, if someone's food smells like "garbage").
In your case, you've made it clear that it's neither a cultural or hygiene issue; it's obviously the morning sickness!
About a solution:
Does anybody know if personal air filters (that operate on batteries or that plug in) really work? I worry about the ionizer ones if they emit ozone, and they probably take a while to kick in, huh?
One product that does work immediately, at least around baby diapers and bathroom odors and smelly shoes, is Zero Odor spray. There's no fragrance to "cover up" the odors, which I do appreciate!