Say God bless you, because they need it.
Give them tissues, because it possibly makes them feel less like crap.
Don't joke about how disgusting it is, unless you think it will make them smile.
If they look bleary eyed, don't say so, unless you add "you poor thing" after that and mean it.
Cos otherwise they ginna glare at you. And then they ginna smack you so hard.
Either that, or they might just feel like sneezing in your face, and "react too slowly".
Get mah drift.
D'arvit.