Wait, are they layering the greens and browns properly or just dumping everything in a pile? Ive seen people mess this up so many times, they think composting is just throwing food in a bin. The citrus and onion thing is real, it can kill off the good microbes if they overdo it. Also, if they skip the browns, that pile will turn into a slimy, stinking mess real quick. Tell them to aim for about 3 parts browns to 1 part greens by volume, that keeps the smell down and the worms happy. And yeah, chopping scraps small makes a huge difference, I learned that the hard way when my bin started smelling like rotten eggs.

Friend of mine spent half a day digging up what he thought was just a broken pipe.

I read somewhere that worms actually have a weird reaction to salt, so maybe that's why the citrus thing works since citrus has some natural salts in the peel. Your mileage may vary but cutting back on the bananas helped my bin a lot.