sounds like your friend depleted his phosphates to 0 which would allow dinos to outbreak since the bacteria were thrown out of the redfield ratio for uptaking nutrients, i can't really recommend a treatment method specifically since to do that it will take a microscope with atleast 100x zoom to identify what strain of the 2400 strains that exist you are actually taking about. so in the meantime i suggest that he adds phosphates back to the tank, either by using sodium phosphate (preferred) or potassium phosphate (freshwater fertilizer phosphate) to get a reading of atleast .1 with nitrates preferably above 10.
now alot of people say their success lies with dosing h2o2 but id advise against that early on, for one simple reason, its bacteria that outcompete dinos in the long run, not microalgaes or macroalgaes. while they can outcompete dinos for available nutrients, its the stabilization of the bacterial colonies that help tanks stabilize in the long run. one method that works really well is the elegant corals regimine for dinoflagellates. i will link it here
https://www.google.com/url?sa=i&url=https://elegantcorals.com/blog/&psig=AOvVaw0bkarIesWUk1qgYSD4Sfvx&ust=1582425104522000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCODXz4OP5OcCFQAAAAAdAAAAABAI
if you go this route you MUST follow every direction absolutely, or the treatment will fail and possibly crash your tank if you misuse any of the dosing materials. however this has worked for me in my personal experience within 7 days all dinos are either severely cut back or diminished altogether.
others use UV sterilization, but again, unless you know you have a free swimming version of dino all youd be doing is killing bacteria/algae that would otherwise compete against the dinos.
it is very important that while you may rid the dinos, you should be checking alk and nutrients specifically to make sure you stay above 7 alk and above 0 nitrate/phos for the weeks following, as any alk lower than 7 and depletion of nutrients will just lead to the problem starting all over again, dinos are always in every system, even zooxenthallae is technically a dinoflagellate, so the goal is to keep parameters stable not necessarily kill dinos.
all the best of luck to your friend