Obviously it's a troll, but still. First of all, HF is not present as a molecule at physiological pH, it's an acid - so it's present as the fluoride anion. Ions (especially hard ones like fluoride) are fairly shit at crossing membranes, including the blood-brain barrier, so that puts a dent in the whole idea. Second of all, EVEN IF it did work, it would require fluoride concentrations way above what we get from drinking water and occasionally brushing our teeth (cause we don't get high from that), and it is known that levels not much higher than those are already harmful. So, if it did work, you'd be dead before you got high.