Vape juice contains Vegetable Glycerin (VG) and Propelyne Glycol (PG). To the best of my knowledge, those are the base ingredients (on different percentages) of all e-juice.
It's very possible that's she coughing a lot from vaping if she is still actively smoking cigarettes as well, cigs burn the cilia in the respiratory tract and the vape irritates the damaged cilia.
But with the juul I have found it very irritating to my throat if I use it a lot. A simple yet very effective fix is just to drink water when it feels irritated
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