Beca, if it's not something you can say directly to your coworker then you probably shouldn't say anything. If you can't say, "Your sniffing is annoying me so would you mind using some tissue"? without feeling as though you are being too forward or rude, then you probably would be out of line. Sorry, you'll probably just have to put up with the other human beings in your office when they sniffle...
Mrs. Earbore of Grosse Pointe