Check the labels.
Nearly every single over the counter lung congestion syrup is the same exact thing.
Guaifenesin.
It's what is in Robotussin, Musinex, Tussin, and more. They all have the same exact med in them. Of course if you have money to spend because you want a name brand...I like Tussin, Walmart's brand, it is about half of the other name brand meds.
Tussin/Guaifenesin is an expectorant. That means it is specifially designed to bring fluid to the lungs, thin lung mucus, so a person can cough it up easier. It is made to MAKE YOU COUGH up goo. Not stop a cough. It's made to make you get that goo out.
Just like the Musinex commercials where the little round family is coughed up. That's what an expectorant is supposed to do, make you cough it up and out.
Google expectorant defined if you need more information.
Have her take several really warm showers every day, it cleans the body of allergens, dust, dander, pollutants, etc...and the moisture will help her lungs.
Runny noses are trying to rinse the nose/sinuses out. That's their job. There is something irritating her inside. Otherwise she wouldn't be runny.
Giving her Benadryl will DRY UP HER LUNGS A LOT. Don't give her any antihistamines. She's struggling to get moisture into her lungs and Benadryl will totally dry up everything, even her saliva.
She may need something to help her mucus rinse out her nose, maybe some Sudafed. I can't take the fake Sudafed, it makes my heart race. I get real Sudafed at the Pharmacy. It expands the blood vessels in my sinuses and that helps the area to open up and let the goo out. This will rinse her sinuses well.
I don't know what to do about the runny nose except get her to blow it all the time. That will help get the stuff out that's bothering her inside. It will also rinse the virus out sooner.