adam42 tough question.
What I would probably do is:
- send an email to their account making them aware about the content of the person on your community, in the hopes a relative reads it and can make a decision whether to keep the content or not
- anonimize their data in the meantime
I would not delete it, there might be posts by this user that their relatives might want to have.
In the end it depends on the type of community and the person as well I guess.