Have they checked their spam and junk folders? I've found a fair few emails from friends using their usual emails have slipped into the the garbage for no good reason.
have you had a reply from Daemon-failure? or whatever it's called?
to let you know that it hasn't been sent?
Have you checked your sent box?
(that sounds rude)
If you have made a simple mistake with the email address, it will either go to another recipient, or it will be returned to you as undeliverable. Sorry if that doesn't answer your question any better.