It’s the same with insect/spider bites and stings and so on… the fear of the unknown has you reaching out for help wherever you can get it from…
I have to resist the temptation to reply “Try eating it yourself, and if you both get sick then it is pretty likely it was, but you could also have both been coming down with the same illness completely unrelated…”, but that of course would be completely irresponsible to do!
My actual response would be that if they are showing signs and symptoms of poisoning, then they should immediately contact a poisons centre, their GP or a hospital, or call emergency services. I think here in NZ a call to emergency services they will put you through to poisons centre after asking some preliminary questions, but I am not sure on that. If they aren’t showing signs and symptoms, but they still have a concern that it might be poisonous, they should place some of the material in a plastic bag and take it (and the ingestor) to their local GP. Otherwise I would suggest watching and monitoring the ingestor for any changes over the next 24 hours and to act accordingly if there are.
The real difficulty with poisonings, is that different people react to different things in different ways. You might recognise the plant, have eaten it before, and give advice that it is safe, but the reality might be that the person has an allergy to it. It should never be looked at as “is this poisonous”, but rather “is this person likely to react adversely to this substance”.