How does Felyne Weakener work in MHW/Iceborne?
In Monster Hunter each quest (or investigation) has set modifiers for health pool, part break defense, spawn location, and a few other variables. Essentially there is very weak, weak, normal, durable, and very durable. Last I read it works the same in MHW/Iceborne as it always has. It gives you a chance to remove durable and very durable type monsters, and you will have a better chance at a 'weaker' monster with lower hp/part defense modifiers.
The simple answer - you have a chance of the monster having less health. This does not stack with other players, and there is always a chance for it to do nothing. Like "Felyne Insurance" (the first faint doesn't count against the quest limit) - "Felyne Weakener" will be activated for that instance. Additional players with the same food perk will not add another free faint or make the chance of a weaker monster higher - it's a quest perk and not a player perk.