The BP can drop because the heart is beating so fast it cannot fill with blood. OR they may be wrong in assuming this is not POTS or SVT. It’s certainly in the range of SVT, especially if it starts suddenly. I’m curious what has been done in the ER for her. Something is seriously wrong, IMO, that this is allowed to continue by her cardiologist. I would consider taking her to the ER of your local university hospital or children’s hospital where someone takes this seriously. I would also have her avoid activities that bring on the tachycardia until this gets figured out. She could pass out, sustain a head injury, fracture, etc. This is dangerous! I hope you get the attention she deserves!!!