Marie,
Please let us know how your daughter's appointment goes later this week.
I wore a monitor for around 45 days, and pressed the button as your daughter has to. Does hers have the "loop back" feature? What that does is, even if she presses the button "x" minute, the loop back has already been recording; mine looped-back 5 mins. After recording, a tele. number is phoned and the recording is transmitted via phone to the "monitoring" center, which has technicians and others interpreting and faxing the readout to the cardiologist.
Unlike a sonogram or ultrasound, the monitoring isn't just for the moments while technicians are checking things.

I think it gives doctors a more in-depth "read" on what's happening.
Mine picked up PVCs and SVTs (along with already dx'd symptomatic MVP).
Until this past year or so, my BP was low. As with your daughter, my cardiologist told me to increase salt intake. One of his ideas is one that, I know, won't work for your daughter: he had to pull a holiday weekend shift and told me to go out and have a margarita with salt for him

. His other idea was eat red meat - along with plenty of fluids, which you already know too well.
When those things weren't working, he put me on the beta blocker Toprol XL, starting at 12.5 MGs a day. Now, I'm on 50 MGs a day and it seems to have really stabilized things. During my cardio. appointments, the doc still detects when things are working/sounding right, yet it's a lot better now than everything felt a couple years ago.
I don't know whether her cardio. doc might have mentioned this, so I'll go ahead and share what mine told me... Whenever I am awake and feeling light-headed or dizzy, sit down immediately if I'm standing - and no matter whether it means I'll have to sit on the floor or ground even in public. (When pain is causing the syncope, the pain is what stops me from standing.)
I can definitely relate to what you and your daughter are going through, Marie.