Monkeys are omnivores like us, so do they have any adaptions to be able to eat fruit that we'd find toxic? Don't know.

Eating something after seeing a herbivore eat it, seems like it'd be a higher risk for sure. All kinds of herbivores have adaptions to eat all kinds of things toxic to us.

Bit like dogs are carnivores, and why some of our foods are toxic to them, as their digestion hasn't adapted due to relatively short time they have been domesticated.