S.C.
I'm so not tactful when it comes to that kind of stuff. Every time she coughs I would say "Cover your mouth please" or "Cover your cough" or "Coughing spreads germs, please cover yours." When she gives you nasty looks say "Oh, I'm sorry..I'm used to telling my kids that all the time." And just keep telling her to cover her cough. Hopefully she'll get it eventually. It's so so so gross and spreads germs all over the place.