Sad to say that I'm never surprised by that sort of attitude from that sort of person.
I'm sorry, what? Where did that come from? I thought this was a forum about geocaching?
Back to topic... I've had a similar experience, but at the end of the day you can't be responsible for someone else's actions. You acted in good faith kewfriend (no pun intendeded), it's just one of those things. You could perhaps try adding a note to the cache to see if someone could look up their name from the logbook.
It came from someone's post that I quoted. You can see it too if you look.