Ok, maybe this isn't such a good idea.
I created a test account on my OS X Lion machine with the password test.
Logged in and made sure it was ok and then logged out.
Then I used the passwd command from my main account on user test and logged back into it.
The password change worked but then Keychain had a problem, you're greeted with a dialog saying -
The System was unable to unlock your login keychain.
Would you like to update the password, create a new keychain or continue the login?
Update Keychain Password is the default and if selected prompts for the keychain password, remember though that this is the old password which it needs and I'm assuming once it's provided, keychain will update itself with the new password.
But at no stage did this process prompt for the user to create a new password of their own.
Of course, all this can be done from System Preferences - User & Groups if you have an Admin account.
Second idea, change their password to something horrid or offensive, then they might change it of their own accord