There is an app called Keychain Access on the mac which allows you to copy a password to the clipboard. If they're using iCloud keychain and they can login to their iCloud account, you might be able to pull it off that way.
This won't work. Yahoo credentials are stored for later use as "XYMCOOKIE" which is some kind of hashed authentication mechanism for SMTP and IMAP. And yes, I tried. I also attempted to monitor SSL communications through SMTP and saw the XYMCOOKIE exchange firsthand. This sucks :-/
That's what I did when the same happened to me. Enable iCloud Keychain on the iPad and the Mac, wait for the passwords to sync (might take a few minutes) and then open Keychain Access on the Mac and find the password on it.
This did not work because the Yahoo password is not stored on the device after being entered. Apparently Yahoo has something called XYMCOOKIE authentication for SMTP and IMAP which is basically just acceptance of a pre-sent hash/cookie. The iPad discards the actual password the first time you enter it and instead stores a XYMCOOKIE.