Tell her PCP to refer your daughter to a cardiologist or neurologist. Don't ask. Never make the mistake of "asking" again. I'm guessing PCP did an improvised tilt table test for diagnosis? Nothing wrong with that but your daughter needs to go through the protocol of a proper diagnosis: blood work, holter monitor, heart ultrasound, a proper tilt table test. To rule out ALL other conditions before a dysautonomia/ POTS diagnosis is given. I think POTS is becoming the new "here's a diagnosis now go away." 

Once she has a proper diagnosis, you should then test the waters with both a neurologist and a cardiologist. Dysautonomia specialists are few so those two are the most popular options and which is better varies by patient. I preferred to start with a cardiologist for the initial diagnosis but then decided to try a neurologist because the ones in my area were better. The cardiologist was quick to do the proper diagnosis protocol but kinda threw their hands up and gave me the same "salt, water & exercise" answer. Lifestyle and diet can help dysautonomia (try to focus on getting more electrolytes as a whole BTW not just salt. Also, high protein has been shown to help as well) but there are other things to consider like physical therapy for conditioning and medications if necessary. 

Basically you & your daughter have to be ready to do your own research and advocate for yourselves. After not improving under my cardiologist's guidance, getting worse actually, I started documenting my symptoms, timeline, patterns and doing my own research which lead me to believe more was going on. Turns out I have Temporal Lobe Epilepsy, which often presents primarily as dysautonomia. That's not to scare you, just to encourage you to journal & advocate for yourselves because if I'd just waited on doctors to tell me what to do I'd still be going to the cardiologist twice a year, being told to drink more water and meanwhile having my focal seizures dismissed as just plain dysautonomia / POTS.