To my mind there is unlikely to be a problem if the person is clearly sick or injured, either then in the presence of witnesses or later. The difficulty is if they are not, they might be drunk, for example, and there are no other witnesses able to back you up. Other witnesses, if there are any, may only have seen half the story and could even give an embroidered version of it. We know that people will often stick to seriously damaging (to others) lies out of a wish not to be seen to have made a mistake.
You are on a train and the only other passenger in your coach is a young woman who suddenly gasps, falls to the floor and lies still. You go to help, realise that she is drunk, but stay to help her get up again. As you are helping her she suddenly loses it and starts screaming at you and calls you a pervert, etc. This attracts passengers from the next coach and she gabbles out an accusation of molestation. What happens next?