I know a few people who have more plaque than teeth. I always want to say something, but it always seems like it'd be rude so I don't.
The plaque isn't really the problem though (well it might be to their dental health but that's on them). It's the breath. I can't stand within 5ft without gagging. Like they're cool people otherwise, and I feel bad that it's so bad, but it literally smells like week old roadkill being near them.
Maybe some flesh eating flies can help here? Put them on the plaque and see if they eat it. The whole "larvae corkscrewing into the flesh" bit might lead to some cavities, but hopefully someone can get that piece under control, and worst case it'll help the dentist prepare for any necessary fillings I guess.