I agree that people have committed suicide by jumping in flight, and I can think of a few other sad stories of this in addition to those referred above, but I believe that those were cases where the passenger sought (and booked/paid for) the flight. If you, as a PPL are inviting a person for a flight (the most likely way that you could have a passenger at all as a PPL) you do not have someone along who has approached you to take them up.
But, yes, were you to be approached by a person not known to you at all, to be taken flying, that should make you think just a little more about your willingness to take that person.