The first thing to remember with a green out is that you can feel like you're dying even though you're not. So first step is to be psychologically supportive - calming, quiet, supportive and nurturing. Reassure people that you won't die from a cannabis overdose, it just feels awful but it will pass. Lots of reminders that it will pass.
THC metabolites are water soluble. You want to flush them out. So drink lots of water. I STRONGLY recommend using electrolytes in water though. Coconut water is great, sugar-free electrolyte drops otherwise. Sports drinks as a last resort. Even just putting a pinch on salt in a glass of water is better than plain water. This is because too much THC can plummet your blood pressure and drinking plain water or getting deficient in electrolytes will make low blood pressure even worse.
You can also drink things that have a diuretic effect to get your body flushing fluids faster. I don't recommend coffee though or anything with large amounts of caffeine because that can make you fee worse. I'm personally fond of rooibos tea. Skip this if someone is vomiting.
CBD can help offset the unpleasant psychological effects of greening out. B-Caryophyllene and limonene terpenes are also great for this (black pepper is high in Caryophyllene which is why it works as a remedy). Think pepper, cloves, rosemary, Oregano, cinnamon, and citrus.
Bonus make a spicy mulled citrus drink (add a little ginger for nausea) that does double duty to help flush out the THC and calms and soothes with its warmth and terpenes.
After the vomiting is past or once someone feels like eating, the usual fatty hangover foods work wonders. 😊