Are you building this for a "Forgot My Password" feature? You could look into using Devise and password recovery is built right in.
If you still want to roll your own Id recommend, at a minimum, you instead reset their password to a random password, email them the random password and let them reset it. That is still not very secure though. Sending any password in plain text defeats the purpose of hashing and storing your passwords. There is several better solutions like having user answer security questions, sending them a link to a temporary obfuscated page that allows them to reset their password, etc. If you're rolling your own right now though I definitely recommend looking into the Devise gem as it will do all this for you.