I think you might suggest they see either a very good ENT doctor or a pediatric allergist.
For all we know, it might or might not be dairy. Might be household allergens, or both.
Constant congestion is serious business. They need to check is she's a mouth breather. Putting a tape recorder at night next to her and then taking the tape to someone who knows what she or he is doing, may be helpful.
It's weird they are completely unaware of dairy allergies, which are number 1 cause of allergies in the US. This is nothing new, one mainstream source I'd browse and get print-outs for them is American Dietetic Association, both for factual information and a practical resource if they decide to adjust their diet . www.eatright.org
Saline drops and checking humidity at home are an absolute must for congested kids, as are exercise, time spent outside etc. Daily book reading with the child on adult's lap, adult sitting on the comode in the bathroom with the door slightly ajar and hot shower running, can also be very helpful. Informed dietary modifications, saline solution, exercise are all safe and might take care of it.
Still I'd suggest taking her to a good wholistic (in the proper sense of the word) practitioner who would look at the whole picture. Neglected case as you describe it might mean ENT surgery down the line, not that it's the end of the world, but better avoided.
I did not understand though what you meant -- she never has clear lungs?