I have already posted on dengue in another thread.
"Tawi tawi tea " is a diuretic ( that is, it can cause peeing) which therefore has the potential to dehydrate. Local doctors may allow patients to drink it - but not "by the gallon". Obviously their advice should always be taken! Prompt and adequate fluid replacement treatment - at home or in hospital - is standard treatment. Dengue affects around 70,000 each year in the Philippines but thankfully the mortality is less than 1% (around 500). In the absence of specific antiviral treatment, prevention of bites by mosquitoes is best where possible.
I have conceded only that "herbal remedies might help" . Many other chemicals derived from plants have, after all, been proved by clinical trials to be effective in treatment of heart disease, cancer, and malaria.
Clinical trials of treatment of humans by tawi tawi have not, however, to my knowledge (or that of the Philippines Department of Health) proved its effectiveness. In a condition where most victims get better with supportive therapy, such a trial proving effectiveness of this or other " herbal remedies " could be difficult - and therefore has so far eluded publication. The onus is not on doctors to prove tawi tawi doesn't work, but for those who believe it does to confirm it convincingly.