Not sure if you want to go thru with this, but you could remove the checkbox to require passwords, go to the user maintenance and remove that user's password, go back and check the box to require passwords. The user will be prompted (forced) to enter a password next time he/she signs on.
Otherwise, I'd do something like @MadelineStefanou suggests, appending ""welcome"" with something that they know, like the last 4 characters of their SS# or MMDD of their birthday.